rev-parse: use selected alternate terms to look up refs

rev-parse: use selected alternate terms to look up refs

git rev-parse --bisect does not work when alternate bisect terms are
used, simply listing no revisions at all.

This is because a such bisect using e.g. "old" and "new" in place of
"good" and "bad" will name refs "refs/bisect/old" (or new) accordingly
so the hardcoded "refs/bisect/bad" (and good) yields no results in a
bisect using alternate terms.

Use the current bisect_terms to make rev-parse --bisect work in an
alternate term bisect.

Signed-off-by: Jonas Rebmann <kernel@schlaraffenlan.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>