refs: change how packed refs are deleted

refs: change how packed refs are deleted

Add a function remove_ref(), which removes a single entry from a
reference cache.

Use this function to reimplement repack_without_ref().  The old
version iterated over all refs, packing all of them except for the one
to be deleted, then discarded the entire packed reference cache.  The
new version deletes the doomed reference from the cache *before*
iterating.

This has two advantages:

* the code for writing packed-refs becomes simpler, because it doesn't
  have to exclude one of the references.

* it is no longer necessary to discard the packed refs cache after
  deleting a reference: symbolic refs cannot be packed, so packed
  references cannot depend on each other, so the rest of the packed
  refs cache remains valid after a reference is deleted.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>