odb: compute object hash in `odb_write_object_ext()`

odb: compute object hash in `odb_write_object_ext()`

Same as in a preceding commit, compute the object hash in
`odb_write_object_ext()` so that we can unify this logic.

Besides unification, this change also allows us to lift the object
existence check out of the "loose" backend into the generic layer, which
will happen in the next commit.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>