built-in rebase --autostash: leave the current branch alone if possible

When we converted a `git reset --hard` call in the original Unix shell
script to built-in code, we asked to reset the worktree and the index
and explicitly *not* to detach the HEAD. By mistake, though, we still
did. Let's fix this.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/rebase.c

@@ -614,7 +614,8 @@ static int reset_head(struct object_id *oid, const char *action,
 		reflog_head = msg.buf;
 	}
 	if (!switch_to_branch)
-		ret = update_ref(reflog_head, "HEAD", oid, orig, REF_NO_DEREF,
+		ret = update_ref(reflog_head, "HEAD", oid, orig,
+				 detach_head ? REF_NO_DEREF : 0,
 				 UPDATE_REFS_MSG_ON_ERR);
 	else {
 		ret = create_symref("HEAD", switch_to_branch, msg.buf);

t/t3420-rebase-autostash.sh

@@ -361,7 +361,7 @@ test_expect_success 'autostash with dirty submodules' '
 	git rebase -i --autostash HEAD
 '
 
-test_expect_failure 'branch is left alone when possible' '
+test_expect_success 'branch is left alone when possible' '
 	git checkout -b unchanged-branch &&
 	echo changed >file0 &&
 	git rebase --autostash unchanged-branch &&