remote: repack packed-refs once when deleting multiple refs

remote: repack packed-refs once when deleting multiple refs

When 'git remote rm' or 'git remote prune' were used in a repository
with many refs, and needed to delete many remote-tracking refs, a lot
of time was spent deleting those refs since for each deleted ref,
repack_without_refs() was called to rewrite packed-refs without just
that deleted ref.

To avoid this, call repack_without_refs() first to repack without all
the refs that will be deleted, before calling delete_ref() to delete
each one completely.  The call to repack_without_ref() in delete_ref()
then becomes a no-op, since packed-refs already won't contain any of
the deleted refs.

Signed-off-by: Jens Lindström <jl@opera.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>