builtin/repack.c: extract common cruft pack loop

builtin/repack.c: extract common cruft pack loop

When generating the list of packs to store in a MIDX (when given the
`--write-midx` option), we include any cruft packs both during
--geometric and non-geometric repacks.

But the rules for when we do and don't have to check whether any of
those cruft packs were queued for deletion differ slightly between the
two cases.

But the two can be unified, provided there is a little bit of extra
detail added in the comment to clarify when it is safe to avoid checking
for any pending deletions (and why it is OK to do so even when not
required).

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