pack-refs: merge code from pack-refs.{c,h} into refs.{c,h}

pack-refs: merge code from pack-refs.{c,h} into refs.{c,h}

pack-refs.c doesn't contain much code, and the code it does contain is
closely related to reference handling.  Moreover, there is some
duplication between pack_refs() and repack_without_ref().  Therefore,
merge pack-refs.c into refs.c and pack-refs.h into refs.h.

The code duplication will be addressed in future commits.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>