docs: add documentation for our coding style

For years, we've repeatedly had confusion about what our actual coding
style is not only for newcomers, but also across the core contributors.
This can mostly be attributed to the fact that we do not have any coding
conventions written down. This is now a thing of the past with the
introduction of a new document that gives an initial overview of our
style and most important best practices for both our C codebase as well
as for CMake.

While the proposed coding style for our C codebase should be rather
uncontroversial, the coding style for CMake might be. This can be
attributed to multiple facts. First, the CMake code base doesn't really
have any uniform coding style and is quite outdated in a lot of places.
Second, the proposed coding style actually breaks with our existing one:
we currently use all-uppercase function names and variables, but the
documented coding style says we use all-lowercase function names but
all-uppercase variables.

It's common practice in CMake to write variables in all upper-case, and
in fact all variables made available by CMake are exactly that. As
variables are case-sensitive in CMake, we cannot and shouldn't break
with this. In contrast, function calls are case insensitive, and modern
CMake always uses all-lowercase ones. I argue we should do the same to
get in line with other codebases and to reduce the likelihood of
repetitive strain injuries.

So especially for CMake, the proposed coding style says something we
don't have yet. I'm fine with that, as the document explicitly says that
it's what we want to have and not what we have right now.

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+# libgit2 Coding Style
+
+This documentation describes the preferred coding style for the libgit2 project.
+While not all parts of our code base conform to this coding style, the outlined
+rules are what we aim for.
+
+Note that in no case do we accept changes that convert huge parts of the code
+base to use our coding style. Instead, it is encouraged to modernize small parts
+of code you're going to modify anyway for a given change you want to introduce.
+A good rule to follow is the Boy Scout Rule: "Leave the campground cleaner than
+you found it."
+
+## C Coding Style
+
+The following sections define the coding style for all code files and headers.
+
+### Indentation and Alignment
+
+Code is indented by tabs, where a tab is 8 spaces. Each opening scope increases
+the indentation level.
+
+```c
+int foobar(int void)
+{
+	if (condition)
+		doit();
+	/* Body */
+}
+```
+
+Switch statements have their `case`s aligned with the `switch` keyword. Case
+bodies are indented by an additional level. Case bodies should not open their
+own scope to declare variables.
+
+```c
+switch (c) {
+case 'a':
+case 'b':
+	return 0;
+default:
+	return -1;
+}
+```
+
+Multi-line conditions should be aligned with the opening brace of the current
+statement:
+
+```c
+if (one_very_long_condition(c) &&
+    another_very_long_condition(c))
+	doit();
+```
+
+### Spaces
+
+There must be no space between the function and its arguments, arguments must be
+separated by a space:
+
+```c
+int doit(int first_arg, int second_arg);
+doit(1, 2);
+```
+
+For any binary or ternary operators, the arguments and separator must be
+separated by a space:
+
+```c
+1 + 2;
+x ? x : NULL;
+```
+
+Unary operators do not have a space between them and the argument they refer to:
+
+```c
+*c
+&c
+```
+
+The `sizeof` operator always must not have a space and must use braces around
+the type:
+
+```
+sizeof(int)
+```
+
+There must be a space after the keywords `if`, `switch`, `case`, `do` and
+`while`.
+
+### Braces
+
+Functions must have their opening brace on the following line:
+
+```c
+void foobar(void)
+{
+	doit();
+}
+```
+
+For conditions, braces should be placed on the same line as the condition:
+
+```c
+if (condition(c)) {
+	doit();
+	dothat();
+}
+
+while (true) {
+	doit();
+}
+```
+
+In case a condition's body has a single line, only, it's allowed to omit braces,
+except if any of its `else if` or `else` branches has more than one line:
+
+```c
+if (condition(c))
+	doit();
+
+if (condition(c))
+	doit();
+else if (other_condition(c))
+	doit();
+
+/* This example must use braces as the `else if` requires them. */
+if (condition(c)) {
+	doit();
+} else if (other_condition(c)) {
+	doit();
+	dothat();
+} else {
+	abort();
+}
+```
+
+### Comments
+
+Comments must use C-style `/* */` comments. C++-style `// `comments are not
+allowed in our codebase. This is a strict requirement as libgit2 tries to be
+compliant with the ISO C90 standard, which only allows C-style comments.
+
+Single-line comments may have their opening and closing tag on the same line:
+
+```c
+/* This is a short comment. */
+```
+
+For multi-line comments, the opening and closing tag should be empty:
+
+```c
+/*
+ * This is a rather long and potentially really unwiedly but informative
+ * multiline comment that helps quite a lot.
+ */
+```
+
+Public functions must have documentation that explain their usage, internal
+functions should have a comment. We use Docurium to generate documentation
+derived from these comments, which uses syntax similar to Doxygen. The first
+line should be a short summary of what the function does. More in-depth
+explanation should be separated from that first line by an empty line.
+Parameters and return values should be documented via `@return` and `@param`
+tags:
+
+```c
+/*
+ * Froznicate the string.
+ *
+ * Froznicate the string by foobaring its internal structure into a more obvious
+ * translation. Note that the returned string is a newly allocated string that
+ * shall be `free`d by the caller.
+ *
+ * @param s String to froznicate
+ * @return A newly allocated string or `NULL` in case an error occurred.
+ * /
+char *froznicate(const char *s);
+```
+
+### Variables
+
+Variables must be declared at the beginning of their scope. This is a strict
+requirement as libgit2 tries to be compliant with the ISO C90 standard, which
+forbids mixed declarations and code:
+
+```c
+void foobar(void)
+{
+	char *c = NULL;
+	int a, b;
+
+	a = 0;
+	b = 1;
+
+	return c;
+}
+```
+
+### Naming
+
+Variables must have all-lowercase names. In case a variable name has multiple
+words, words should be separated by an underscore `_` character. While
+recommended to use descriptive naming, common variable names like `i` for
+indices are allowed.
+
+All public functions must have a `git` prefix as well as a prefix indicating
+their respective subsystem. E.g. a function that opens a repository should be
+called `git_repository_open()`. Functions that are not public but declared in
+an internal header file for use by other subsystems should follow the same
+naming pattern. File-local static functions must not have a `git` prefix, but
+should have a prefix indicating their respective subsystem.
+
+All structures declared in the libgit2 project must have a `typedef`, we do not
+use `struct type` variables. Type names follow the same schema as functions.
+
+### Error Handling
+
+The libgit2 project mostly uses error codes to indicate errors. Error codes are
+always of type `int`, where `0` indicates success and a negative error code
+indicates an error case. In some cases, positive error codes may be used to
+indicate special cases. Returned values that are not an error code should be
+returned via an out parameter. Out parameters must always come first in the list
+of arguments.
+
+```c
+int doit(const char **out, int arg)
+{
+	if (!arg)
+		return -1;
+	*out = "Got an argument";
+	return 0;
+}
+```
+
+To avoid repetitive and fragile error handling in case a function has resources
+that need to be free'd, we use `goto out`s:
+
+```c
+int doit(char **out, int arg)
+{
+	int error = 0;
+	char *c;
+
+	c = malloc(strlen("Got an argument") + 1);
+	if (!c) {
+		error = -1;
+		goto out;
+	}
+
+	if (!arg) {
+		error = -1;
+		goto out;
+	}
+
+	strcpy(c, "Got an argument")
+	*out = c;
+
+out:
+	if (error)
+		free(c);
+	return error;
+}
+```
+
+When calling functions that return an error code, you should assign the error
+code to an `error` variable and, in case an error case is indicated and no
+custom error handling is required, return that error code:
+
+```c
+int foobar(void)
+{
+	int error;
+
+	if ((error = doit()) < 0)
+		return error;
+
+	return 0;
+}
+```
+
+When doing multiple function calls where all of the functions return an error
+code, it's common practice to chain these calls together:
+
+```c
+int doit(void)
+{
+	int error;
+
+	if ((error = dothis()) < 0 ||
+	    (error = dothat()) < 0)
+		return error;
+
+	return 0;
+}
+```
+
+## CMake Coding Style
+
+The following section defines the coding style for our CMake build system.
+
+### Indentation
+
+Code is indented by tabs, where a tab is 8 spaces. Each opening scope increases
+the indentation level.
+
+```cmake
+if(CONDITION)
+	doit()
+endif()
+```
+
+### Spaces
+
+There must be no space between keywords and their opening brace. While this is
+the same as in our C codebase for function calls, this also applies to
+conditional keywords. This is done to avoid the awkward-looking `else ()`
+statement.
+
+```cmake
+if(CONDITION)
+	doit()
+else()
+	dothat()
+endif()
+```
+
+### Case
+
+While CMake is completely case-insensitive when it comes to function calls, we
+want to agree on a common coding style for this. To reduce the danger of
+repetitive strain injuries, all function calls should be lower-case (NB: this is
+not currently the case yet, but introduced as a new coding style by this
+document).
+
+Variables are written all-uppercase. In contrast to functions, variables are
+case-sensitive in CMake. As CMake itself uses upper-case variables in all
+places, we should follow suit and do the same.
+
+Control flow keywords must be all lowercase. In contrast to that, test keywords
+must be all uppercase:
+
+```cmake
+if(NOT CONDITION)
+        doit()
+elseif(FOO AND BAR)
+        dothat()
+endif()
+```
+
+### Targets
+
+CMake code should not use functions that modify the global scope but prefer
+their targeted equivalents, instead. E.g. instead of using
+`include_directories()`, you must use `target_include_directories()`. An
+exception to this rule is setting up global compiler flags like warnings or
+flags required to set up the build type.
+
+### Dependencies
+
+Dependencies should not be discovered or set up in the main "CMakeLists.txt"
+module. Instead, they should either have their own module in our top-level
+"cmake/" directory or have a "CMakeLists.txt" in their respective "deps/"
+directory in case it is a vendored library. All dependencies should expose
+interface library targets that can be linked against with
+`target_link_libraries()`.