builtin/repack.c: keep track of existing packs unconditionally

builtin/repack.c: keep track of existing packs unconditionally

In order to be able to write a multi-pack index during repacking, `git
repack` must keep track of which packs it wants to write into the MIDX.
This set is the union of existing packs which will not be deleted,
new pack(s) generated as a result of the repack, and .keep packs.

Prior to this patch, `git repack` populated the list of existing packs
only when repacking all-into-one (i.e., with `-A` or `-a`), but we will
soon need to know this list when repacking when writing a MIDX without
a-i-o.

Populate the list of existing packs unconditionally, and guard removing
packs from that list only when repacking a-i-o.

Additionally, keep track of filenames of kept packs separately, since
this, too, will be used in an upcoming patch.

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