builtin/repack.c: extract redundant pack cleanup for existing packs

To remove redundant packs at the end of a repacking operation, Git uses its `remove_redundant_pack()` function in a loop over the set of pre-existing, non-kept packs. In a later commit, we will split this list into two, one for pre-existing cruft pack(s), and another for non-cruft pack(s). Prepare for this by factoring out the routine to loop over and delete redundant packs into its own function. Instead of calling `remove_redundant_pack()` directly, we now will call `remove_redundant_existing_packs()`, which itself dispatches a call to `remove_redundant_packs_1()`. Note that the geometric repacking code will still call `remove_redundant_pack()` directly, but see the previous commit for more details. Having `remove_redundant_packs_1()` exist as a separate function may seem like overkill in this patch. However, a later patch will call `remove_redundant_packs_1()` once over two separate lists, so this refactoring sets us up for that. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>