builtin/repack.c: avoid "the_repository" in existing packs API

builtin/repack.c: avoid "the_repository" in existing packs API

There are a number of spots within builtin/repack.c which refer to
"the_repository", and either make use of the "existing packs" API
or otherwise have a 'struct existing_packs *' in scope.

Add a "repo" member to "struct existing_packs" and use that instead of
"the_repository" in such locations.

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