builtin/pack-refs: release allocated memory

builtin/pack-refs: release allocated memory

Some of the command line options in `cmd_pack_refs()` require us to
allocate memory. This memory is never released and thus leaking, but we
paper over this leak by declaring the respective variables as `static`
function-level variables, which is somewhat awkward.

Refactor the code to release the allocated memory and drop the `static`
declaration. While at it, remove the useless `flags` variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>