builtin/repack.c: avoid "the_hash_algo" when deleting packs

builtin/repack.c: avoid "the_hash_algo" when deleting packs

The "mark_packs_for_deletion_1" function uses "the_hash_algo->hexsz" to
isolate a pack's checksum before deleting it to avoid deleting a newly
written pack having the same checksum (that is, some generated pack
wound up identical to an existing pack).

Avoid this by passing down a "struct git_hash_algo" pointer, and refer to
the hash algorithm through it instead.

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