The flag GIT_FILEBUF_FORCE currently does two things:
     1. It will cause the filebuf to create non-existing leading
        directories for the file that is about to be written.
     2. It will forcibly remove any pre-existing locks.
While most call sites actually do want (1), they do not want to
remove pre-existing locks, as that renders the locking mechanisms
effectively useless.
Introduce a new flag `GIT_FILEBUF_CREATE_LEADING_DIRS` to
separate both behaviours cleanly from each other and convert
callers to use it instead of `GIT_FILEBUF_FORCE` to have them
honor locked files correctly.

As this conversion removes all current users of `GIT_FILEBUF_FORCE`,
this commit removes the flag altogether.

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.

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.

fsync all the things

The symlink destination is always concatenated to the original path. Fix
this by using `git_buf_sets` instead of `git_buf_puts`.

When fsync'ing files, fsync the parent directory in the case where we
rename a file into place, or create a new file, to ensure that the
directory entry is flushed correctly.