We're missing some header includes leading to missing function
prototypes. While we currently don't warn about these, we should have
their respective headers included in order to detect the case where a
function signature change results in an incompatibility.

We have some functions which aren't used anywhere. Let's remove them to
get rid of unneeded baggage.

Currently, the reflog disallows any entries that have a message with
newlines, as that would effectively break the reflog format, which may
contain a single line per entry, only. Upstream git behaves a bit
differently, though, especially when considering stashes: instead of
rejecting any reflog entry with newlines, git will simply replace
newlines with spaces. E.g. executing 'git stash push -m "foo\nbar"' will
create a reflog entry with "foo bar" as entry message.

This commit adjusts our own logic to stop rejecting commit messages with
newlines. Previously, this logic was part of `git_reflog_append`, only.
There is a second place though where we add reflog entries, which is the
serialization code in the filesystem refdb. As it didn't contain any
sanity checks whatsoever, the refdb would have been perfectly happy to
write malformatted reflog entries to the disk. This is being fixed with
the same logic as for the reflog itself.

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

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

This changes git_signature_dup to actually honor oom conditions raised by
the call to git__strdup. It also aligns it with the error code return
pattern used everywhere else.