builtin/repack.c: avoid "the_hash_algo" in `write_oid()`

builtin/repack.c: avoid "the_hash_algo" in `write_oid()`

In a similar spirit as the previous commit, avoid referring directly to
"the_hash_algo" within builtin/repack.c::write_oid().

Unlike the previous commit, we are within a callback function, so must
introduce a new struct to pass additional data through its "data"
pointer.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>