pull: ff --rebase --autostash works in dirty repo

When `git pull --rebase --autostash` in a dirty repository resulted in a
fast-forward, nothing was being autostashed and the pull failed. This
was due to a shortcut to avoid running rebase when we can fast-forward,
but autostash is ignored on that codepath.

Now we will only take the shortcut if autostash is not in effect.
Based on a few tests against the git.git repo, the shortcut does not
seem to give us significant performance benefits, on Linux at least.
Regardless, it is more important to be correct than to be fast.

Signed-off-by: Tyler Brazier <tyler@tylerbrazier.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pull.c

@@ -772,6 +772,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	struct oid_array merge_heads = OID_ARRAY_INIT;
 	struct object_id orig_head, curr_head;
 	struct object_id rebase_fork_point;
+	int autostash;
 
 	if (!getenv("GIT_REFLOG_ACTION"))
 		set_reflog_message(argc, argv);
@@ -800,8 +801,8 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	if (!opt_rebase && opt_autostash != -1)
 		die(_("--[no-]autostash option is only valid with --rebase."));
 
+	autostash = config_autostash;
 	if (opt_rebase) {
-		int autostash = config_autostash;
 		if (opt_autostash != -1)
 			autostash = opt_autostash;
 
@@ -862,16 +863,18 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 		die(_("Cannot rebase onto multiple branches."));
 
 	if (opt_rebase) {
-		struct commit_list *list = NULL;
-		struct commit *merge_head, *head;
+		if (!autostash) {
+			struct commit_list *list = NULL;
+			struct commit *merge_head, *head;
 
-		head = lookup_commit_reference(orig_head.hash);
-		commit_list_insert(head, &list);
-		merge_head = lookup_commit_reference(merge_heads.oid[0].hash);
-		if (is_descendant_of(merge_head, list)) {
-			/* we can fast-forward this without invoking rebase */
-			opt_ff = "--ff-only";
-			return run_merge();
+			head = lookup_commit_reference(orig_head.hash);
+			commit_list_insert(head, &list);
+			merge_head = lookup_commit_reference(merge_heads.oid[0].hash);
+			if (is_descendant_of(merge_head, list)) {
+				/* we can fast-forward this without invoking rebase */
+				opt_ff = "--ff-only";
+				return run_merge();
+			}
 		}
 		return run_rebase(&curr_head, merge_heads.oid, &rebase_fork_point);
 	} else {

t/t5520-pull.sh

@@ -272,6 +272,24 @@ test_expect_success '--rebase fast forward' '
 	test_cmp reflog.expected reflog.fuzzy
 '
 
+test_expect_success '--rebase --autostash fast forward' '
+	test_when_finished "
+		git reset --hard
+		git checkout to-rebase
+		git branch -D to-rebase-ff
+		git branch -D behind" &&
+	git branch behind &&
+	git checkout -b to-rebase-ff &&
+	echo another modification >>file &&
+	git add file &&
+	git commit -m mod &&
+
+	git checkout behind &&
+	echo dirty >file &&
+	git pull --rebase --autostash . to-rebase-ff &&
+	test "$(git rev-parse HEAD)" = "$(git rev-parse to-rebase-ff)"
+'
+
 test_expect_success '--rebase with conflicts shows advice' '
 	test_when_finished "git rebase --abort; git checkout -f to-rebase" &&
 	git checkout -b seq &&