object-file: update the loose object map when writing loose objects

object-file: update the loose object map when writing loose objects

To implement SHA1 compatibility on SHA256 repositories the loose
object map needs to be updated whenver a loose object is written.
Updating the loose object map this way allows git to support
the old hash algorithm in constant time.

The functions write_loose_object, and stream_loose_object are
the only two functions that write to the loose object store.

Update stream_loose_object to compute the compatibiilty hash, update
the loose object, and then call repo_add_loose_object_map to update
the loose object map.

Update write_object_file_flags to convert the object into
it's compatibility encoding, hash the compatibility encoding,
write the object, and then update the loose object map.

Update force_object_loose to lookup the hash of the compatibility
encoding, write the loose object, and then update the loose object
map.

Update write_object_file_literally to convert the object into it's
compatibility hash encoding, hash the compatibility enconding, write
the object, and then update the loose object map, when the type string
is a known type.  For objects with an unknown type this results in a
partially broken repository, as the objects are not mapped.

The point of write_object_file_literally is to generate a partially
broken repository for testing.  For testing skipping writing the loose
object map is much more useful than refusing to write the broken
object at all.

Except that the loose objects are updated before the loose object map
I have not done any analysis to see how robust this scheme is in the
event of failure.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>