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>

builtin/rebase.c

@@ -1769,6 +1769,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	strbuf_addf(&msg, "%s: checkout %s",
 		    getenv(GIT_REFLOG_ACTION_ENVIRONMENT), options.onto_name);
 	ropts.oid = &options.onto->object.oid;
+	ropts.orig_head = &options.orig_head,
 	ropts.flags = RESET_HEAD_DETACH | RESET_ORIG_HEAD |
 			RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
 	ropts.head_msg = msg.buf;

reset.c

@@ -15,6 +15,7 @@ static int update_refs(const struct reset_head_opts *opts,
 	unsigned detach_head = opts->flags & RESET_HEAD_DETACH;
 	unsigned run_hook = opts->flags & RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
 	unsigned update_orig_head = opts->flags & RESET_ORIG_HEAD;
+	const struct object_id *orig_head = opts->orig_head;
 	const char *switch_to_branch = opts->branch;
 	const char *reflog_branch = opts->branch_msg;
 	const char *reflog_head = opts->head_msg;
@@ -43,7 +44,8 @@ static int update_refs(const struct reset_head_opts *opts,
 				strbuf_addstr(&msg, "updating ORIG_HEAD");
 				reflog_orig_head = msg.buf;
 			}
-			update_ref(reflog_orig_head, "ORIG_HEAD", head,
+			update_ref(reflog_orig_head, "ORIG_HEAD",
+				   orig_head ? orig_head : head,
 				   old_orig, 0, UPDATE_REFS_MSG_ON_ERR);
 		} else if (old_orig)
 			delete_ref(NULL, "ORIG_HEAD", old_orig, 0);

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.
 	 */

t/t3418-rebase-continue.sh

@@ -308,4 +308,30 @@ test_expect_success 'there is no --no-reschedule-failed-exec in an ongoing rebas
 	test_expect_code 129 git rebase --edit-todo --no-reschedule-failed-exec
 '
 
+test_orig_head_helper () {
+	test_when_finished 'git rebase --abort &&
+		git checkout topic &&
+		git reset --hard commit-new-file-F2-on-topic-branch' &&
+	git update-ref -d ORIG_HEAD &&
+	test_must_fail git rebase "$@" &&
+	test_cmp_rev ORIG_HEAD commit-new-file-F2-on-topic-branch
+}
+
+test_orig_head () {
+	type=$1
+	test_expect_success "rebase $type sets ORIG_HEAD correctly" '
+		git checkout topic &&
+		git reset --hard commit-new-file-F2-on-topic-branch &&
+		test_orig_head_helper $type main
+	'
+
+	test_expect_success "rebase $type <upstream> <branch> sets ORIG_HEAD correctly" '
+		git checkout main &&
+		test_orig_head_helper $type main topic
+	'
+}
+
+test_orig_head --apply
+test_orig_head --merge
+
 test_done