worktree list: keep the list sorted

It makes it easier to write tests for. But it should also be good for
the user since locating a worktree by eye would be easier once they
notice this.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -447,7 +447,7 @@ static int list(int ac, const char **av, const char *prefix)
 	if (ac)
 		usage_with_options(worktree_usage, options);
 	else {
-		struct worktree **worktrees = get_worktrees(0);
+		struct worktree **worktrees = get_worktrees(GWT_SORT_LINKED);
 		int path_maxlen = 0, abbrev = DEFAULT_ABBREV, i;
 
 		if (!porcelain)

t/t2027-worktree-list.sh

@@ -117,4 +117,23 @@ test_expect_success 'broken main worktree still at the top' '
 	)
 '
 
+test_expect_success 'linked worktrees are sorted' '
+	mkdir sorted &&
+	git init sorted/main &&
+	(
+		cd sorted/main &&
+		test_tick &&
+		test_commit new &&
+		git worktree add ../first &&
+		git worktree add ../second &&
+		git worktree list --porcelain | grep ^worktree >actual
+	) &&
+	cat >expected <<-EOF &&
+	worktree $(pwd)/sorted/main
+	worktree $(pwd)/sorted/first
+	worktree $(pwd)/sorted/second
+	EOF
+	test_cmp expected sorted/main/actual
+'
+
 test_done

worktree.c

@@ -160,6 +160,13 @@ static void mark_current_worktree(struct worktree **worktrees)
 	free(git_dir);
 }
 
+static int compare_worktree(const void *a_, const void *b_)
+{
+	const struct worktree *const *a = a_;
+	const struct worktree *const *b = b_;
+	return fspathcmp((*a)->path, (*b)->path);
+}
+
 struct worktree **get_worktrees(unsigned flags)
 {
 	struct worktree **list = NULL;
@@ -191,6 +198,13 @@ struct worktree **get_worktrees(unsigned flags)
 	ALLOC_GROW(list, counter + 1, alloc);
 	list[counter] = NULL;
 
+	if (flags & GWT_SORT_LINKED)
+		/*
+		 * don't sort the first item (main worktree), which will
+		 * always be the first
+		 */
+		QSORT(list + 1, counter - 1, compare_worktree);
+
 	mark_current_worktree(list);
 	return list;
 }

worktree.h

@@ -15,6 +15,8 @@ struct worktree {
 
 /* Functions for acting on the information about worktrees. */
 
+#define GWT_SORT_LINKED (1 << 0) /* keeps linked worktrees sorted */
+
 /*
  * Get the worktrees.  The primary worktree will always be the first returned,
  * and linked worktrees will be pointed to by 'next' in each subsequent