rebase --apply: set ORIG_HEAD correctly

At the start of a rebase, ORIG_HEAD is updated to the tip of the
branch being rebased. Unfortunately reset_head() always uses the
current value of HEAD for this which is incorrect if the rebase is
started with "git rebase <upstream> <branch>" as in that case
ORIG_HEAD should be updated to <branch>. This only affects the "apply"
backend as the "merge" backend does not yet use reset_head() for the
initial checkout. Fix this by passing in orig_head when calling
reset_head() and add some regression tests.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reset.h

@@ -22,6 +22,10 @@ struct reset_head_opts {
 	 * The commit to checkout/reset to. Defaults to HEAD.
 	 */
 	const struct object_id *oid;
+	/*
+	 * Optional value to set ORIG_HEAD. Defaults to HEAD.
+	 */
+	const struct object_id *orig_head;
 	/*
 	 * Optional branch to switch to.
 	 */