builtin/repack.c: pass "packdir" when removing packs

builtin/repack.c: pass "packdir" when removing packs

builtin/repack.c defines a static "packdir" to instruct pack-objects on
where to write any new packfiles. This is also the directory scanned
when removing any packfiles which were made redundant by the latest
repack.

Prepare to move the "existing_packs_remove_redundant" function to its
own compilation unit by passing in this information as a parameter to
that function.

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