odb: compute compat object ID in `odb_write_object_ext()`

odb: compute compat object ID in `odb_write_object_ext()`

Repositories can have a compatibility hash configured, which means that
such a repository is expected to maintain a mapping between canonical
and compatibility object hashes. Maintaining this mapping is the
responsibility of the object database sources, where we either store
them as part of the loose objects map or in packfile indices v3 (once we
gain support for this feature).

But besides storing these compatibility hashes, the sources are also
responsible for generating the compatibility hash in the first place.
This is somewhat unnecessary though, as the compatibility hash should be
computed the same no matter which source is being used. The consequence
is that we need to duplicate this functionality across the different
backends, which does not make a lot of sense.

Refactor the code so that we instead compute the compatibility hash in
`odb_write_object_ext()` and then pass the computed value to the
sources. No callers need adjustment as there are none that write objects
via the source interfaces directly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>