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.

Only use defaults for `git_futils_writebuffer` when flags == 0, lest
(1 << 31) be treated as the defaults.

Add a custom `O_FSYNC` bit (if it's not been defined by the operating
system`) so that `git_futils_writebuffer` can optionally do an `fsync`
when it's done writing.

We call `fsync` ourselves, even on systems that define `O_FSYNC` because
its definition is no guarantee of its actual support.  Mac, for
instance, defines it but doesn't support it in an `open(2)` call.

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

Introduce a simple counter that `p_fsync` implements.  This is useful
for ensuring that `p_fsync` is called when we expect it to be, for
example when we have enabled an odb backend to perform `fsync`s when
writing objects.