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.

Worktree fixes

fsync all the things

References for a repository are usually created inside of its gitdir.
When using worktrees, though, these references are not to be created
inside the worktree gitdir, but instead inside the gitdir of its parent
repository, which is the commondir. Like this, branches will still be
available after the worktree itself has been deleted.

The filesystem refdb currently still creates new references inside of
the gitdir. Fix this and have it create references in commondir.

Rename `GIT_OPT_ENABLE_SYNCHRONIZED_OBJECT_CREATION` ->
`GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION`.