libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

If an included config is refreshed twice, the second update is not taken
into account.

This is because the list of included files is cleared after re-reading
the new configuration, instead of being cleared before.

Fix it and add a test case to check for this bug.

Add a few tests that verify some behaviour centered around includes. The
first set of tests verifies that we correctly override values depending
on the order of includes and other keys, the second set asserts that we
can correctly snapshot configuration files with includes.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

Move to the `git_error` name in the internal API for error-related
functions.

In case a configuration includes a key "include.path=" without any
value, the generated configuration entry will have its value set to
`NULL`. This is unexpected by the logic handling includes, and as soon
as we try to calculate the included path we will unconditionally
dereference that `NULL` pointer and thus segfault.

Fix the issue by returning early in both `parse_include` and
`parse_conditional_include` in case where the `file` argument is `NULL`.
Add a test to avoid future regression.

The issue has been found by the oss-fuzz project, issue 10810.

While our tests in config::include create a plethora of configuration
files, most of them do not get removed at the end of each test. This can
cause weird interactions with tests that are being run at a later stage
if these later tests try to create files or directories with the same
name as any of the created configuration files.

Fix the issue by unlinking all created files at the end of these tests.

Modifying variables pulled in by an included file currently succeeds,
but it doesn't actually do what one would expect, as refreshing the
configuration will cause the values to reappear. As we are currently not
really able to support this use case, we will instead just return an
error for deleting and setting variables which were included via an
include.

Right now, we have multiple call sites which initialize a `reader`
structure. As the structure is only actually used inside of
`config_read`, we can instead just move the reader inside of the
`config_read` function. Instead, we can just pass in the configuration
file into `config_read`, which eases code readability.