repack: refactor pack deletion for future use

repack: refactor pack deletion for future use

The repack builtin deletes redundant pack-files and their
associated .idx, .promisor, .bitmap, and .keep files. We will want
to re-use this logic in the future for other types of repack, so
pull the logic into 'unlink_pack_path()' in packfile.c.

The 'ignore_keep' parameter is enabled for the use in repack, but
will be important for a future caller.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>