pack-objects: call release_revisions() after cruft traversal

pack-objects: call release_revisions() after cruft traversal

enumerate_and_traverse_cruft_objects() initializes a rev_info on the
stack but never calls release_revisions() afterwards.  This is not
visible on master but becomes a leak once the revision walking
machinery uses dynamically allocated structures.

Add the missing release_revisions() call.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>