builtin/rebase.c: make sure the active branch isn't moved when autostashing

Consider the following scenario:
    git checkout not-the-master
    work work work
    git rebase --autostash upstream master

Here 'rebase --autostash <upstream> <branch>' incorrectly moves the
active branch (not-the-master) to master (before the rebase).

The expected behavior: (5879477:/git-rebase.sh:526)
    AUTOSTASH=$(git stash create autostash)
    git reset --hard
    git checkout master
    git rebase upstream
    git stash apply $AUTOSTASH

The actual behavior: (6defce2:/builtin/rebase.c:1062)
    AUTOSTASH=$(git stash create autostash)
    git reset --hard master
    git checkout master
    git rebase upstream
    git stash apply $AUTOSTASH

This commit reinstates the 'legacy script' behavior as introduced with
5879477: rebase: implement --[no-]autostash and rebase.autostash

Signed-off-by: Ben Wijen <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bwijen

builtin/rebase.c

@@ -1968,9 +1968,12 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				state_dir_path("autostash", &options);
 			struct child_process stash = CHILD_PROCESS_INIT;
 			struct object_id oid;
-			struct commit *head =
-				lookup_commit_reference(the_repository,
-							&options.orig_head);
+			struct object_id head_oid;
+			struct commit *head;
+
+			if (get_oid("HEAD", &head_oid))
+				die(_("could not determine HEAD revision"));
+			head = lookup_commit_reference(the_repository, &head_oid);
 
 			argv_array_pushl(&stash.args,
 					 "stash", "create", "autostash", NULL);

t/t3420-rebase-autostash.sh

@@ -306,4 +306,12 @@ test_expect_success 'branch is left alone when possible' '
 	test unchanged-branch = "$(git rev-parse --abbrev-ref HEAD)"
 '
 
+test_expect_success 'never change active branch' '
+	git checkout -b not-the-feature-branch unrelated-onto-branch &&
+	test_when_finished "git reset --hard && git checkout master" &&
+	echo changed >file0 &&
+	git rebase --autostash not-the-feature-branch feature-branch &&
+	test_cmp_rev not-the-feature-branch unrelated-onto-branch
+'
+
 test_done