Bisect visualize: use "for-each-ref" to list all good refs.

Bisect visualize: use "for-each-ref" to list all good refs.

In bisect_visualize, "cd $GIT_DIR/refs && echo bisect/good-*" was
still used instead of "git for-each-ref". This patch fix it.

We now pass "refs/bisect/bad" and "--not refs/bisect/good-<rev>"
instead of "bisect/bad" and "--not bisect/good-<rev>" to gitk,
but it seems to work.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>