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

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

In a similar spirit as previous commits, avoid referring directly to
"the_hash_algo" in builtin/repack.c::finish_pack_objects_cmd() and
instead accept one as a parameter to the function.

Since this function has a number of callers throughout the builtin, the
diff is a little noisier than previous commits. However, each hunk is
limited to passing the hash_algo parameter from a repository pointer
that is already in scope.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>