Use the new object_type enumeration names within the codebase.

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.

Rename `GIT_OPT_ENABLE_SYNCHRONIZED_OBJECT_CREATION` ->
`GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION`.

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.

The git_odb_exists_prefix API was not dealing correctly when a
later backend returned GIT_ENOTFOUND even if an earlier backend
had found the object.

Additionally, the unit tests were not properly exercising the API
and had a couple mistakes in checking the results.

Lastly, since the backends are not expected to behavior correctly
unless all bytes of the short id are zero except for the prefix,
this makes the ODB prefix APIs explicitly clear out the extra
bytes so the user doesn't have to be as careful.