built-in rebase: set ORIG_HEAD just once, before the rebase

Technically, the scripted version set ORIG_HEAD only in two spots (which
really could have been one, because it called `git checkout $onto^0` to
start the rebase and also if it could take a shortcut, and in both cases
it called `git update-ref $orig_head`).

Practically, it *implicitly* reset ORIG_HEAD whenever `git reset --hard`
was called.

However, what we really want is that it is set exactly once, at the
beginning of the rebase.

So let's do that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -369,6 +369,7 @@ static void add_var(struct strbuf *buf, const char *name, const char *value)
 #define RESET_HEAD_DETACH (1<<0)
 #define RESET_HEAD_HARD (1<<1)
 #define RESET_HEAD_REFS_ONLY (1<<2)
+#define RESET_ORIG_HEAD (1<<3)
 
 static int reset_head(struct object_id *oid, const char *action,
 		      const char *switch_to_branch, unsigned flags,
@@ -377,6 +378,7 @@ static int reset_head(struct object_id *oid, const char *action,
 	unsigned detach_head = flags & RESET_HEAD_DETACH;
 	unsigned reset_hard = flags & RESET_HEAD_HARD;
 	unsigned refs_only = flags & RESET_HEAD_REFS_ONLY;
+	unsigned update_orig_head = flags & RESET_ORIG_HEAD;
 	struct object_id head_oid;
 	struct tree_desc desc[2] = { { NULL }, { NULL } };
 	struct lock_file lock = LOCK_INIT;
@@ -453,18 +455,21 @@ reset_head_refs:
 	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
 	prefix_len = msg.len;
 
-	if (!get_oid("ORIG_HEAD", &oid_old_orig))
-		old_orig = &oid_old_orig;
-	if (!get_oid("HEAD", &oid_orig)) {
-		orig = &oid_orig;
-		if (!reflog_orig_head) {
-			strbuf_addstr(&msg, "updating ORIG_HEAD");
-			reflog_orig_head = msg.buf;
-		}
-		update_ref(reflog_orig_head, "ORIG_HEAD", orig, old_orig, 0,
-			   UPDATE_REFS_MSG_ON_ERR);
-	} else if (old_orig)
-		delete_ref(NULL, "ORIG_HEAD", old_orig, 0);
+	if (update_orig_head) {
+		if (!get_oid("ORIG_HEAD", &oid_old_orig))
+			old_orig = &oid_old_orig;
+		if (!get_oid("HEAD", &oid_orig)) {
+			orig = &oid_orig;
+			if (!reflog_orig_head) {
+				strbuf_addstr(&msg, "updating ORIG_HEAD");
+				reflog_orig_head = msg.buf;
+			}
+			update_ref(reflog_orig_head, "ORIG_HEAD", orig,
+				   old_orig, 0, UPDATE_REFS_MSG_ON_ERR);
+		} else if (old_orig)
+			delete_ref(NULL, "ORIG_HEAD", old_orig, 0);
+	}
+
 	if (!reflog_head) {
 		strbuf_setlen(&msg, prefix_len);
 		strbuf_addstr(&msg, "updating HEAD");
@@ -1725,7 +1730,7 @@ 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);
 	if (reset_head(&options.onto->object.oid, "checkout", NULL,
-		       RESET_HEAD_DETACH, NULL, msg.buf))
+		       RESET_HEAD_DETACH | RESET_ORIG_HEAD, NULL, msg.buf))
 		die(_("Could not detach HEAD"));
 	strbuf_release(&msg);
 

t/t3400-rebase.sh

@@ -59,7 +59,7 @@ test_expect_success 'rebase against master' '
 	git rebase master
 '
 
-test_expect_failure 'rebase sets ORIG_HEAD to pre-rebase state' '
+test_expect_success 'rebase sets ORIG_HEAD to pre-rebase state' '
 	git checkout -b orig-head topic &&
 	pre="$(git rev-parse --verify HEAD)" &&
 	git rebase master &&