Ensure packfiles with different contents have different names

Initially, the setting has been solely used to enable the use of
`fsync()` when creating objects. Since then, the use has been extended
to also cover references and index files. As the option is not yet part
of any release, we can still correct this by renaming the option to
something more sensible, indicating not only correlation to objects.

This commit renames the option to `GIT_OPT_ENABLE_FSYNC_GITDIR`. We also
move the variable from the object to repository source code.

Upstream git.git has changed the way how packfiles are named.
Previously, they were using a hash of the contained object's OIDs, which
has then been changed to use the hash of the complete packfile instead.
See 1190a1acf (pack-objects: name pack files after trailer hash,
2013-12-05) in the git.git repository for more information on this
change.

This commit changes our logic to match the behavior of core git.

Windows doesn't support it.

Rename `GIT_OPT_ENABLE_SYNCHRONIZED_OBJECT_CREATION` ->
`GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION`.

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.

Honor `git_object__synchronized_writing` when creating a packfile and
corresponding index.