clone: Supply the right commit hash to post-checkout when -b is used

clone: Supply the right commit hash to post-checkout when -b is used

When we use -b <branch>, we may checkout something else than what the
remote's HEAD references, but we still used remote_head to supply the
new ref value to the post-checkout hook, which is wrong.

So instead of using remote_head to find the value to be passed to the
post-checkout hook, we have to use our_head_points_at, which is always
correctly setup, even if -b is not used.

This also fixes a segfault when "clone -b <branch>" is used with a
remote repo that doesn't have a valid HEAD, as in such a case
remote_head is NULL, but we still tried to access it.

Reported-by: Devin Cofer <ranguvar@archlinux.us>
Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>