worktree move: refuse to move worktrees with submodules

Submodules contains .git files with relative paths. After a worktree
move, these files need to be updated or they may point to nowhere.

This is a bandage patch to make sure "worktree move" don't break
people's worktrees by accident. When .git file update code is in
place, this validate_no_submodules() could be removed.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -79,7 +79,7 @@ with `--reason`.
 move::
 
 Move a working tree to a new location. Note that the main working tree
-cannot be moved.
+or linked working trees containing submodules cannot be moved.
 
 prune::
 

builtin/worktree.c

@@ -606,6 +606,27 @@ static int unlock_worktree(int ac, const char **av, const char *prefix)
 	return ret;
 }
 
+static void validate_no_submodules(const struct worktree *wt)
+{
+	struct index_state istate = { NULL };
+	int i, found_submodules = 0;
+
+	if (read_index_from(&istate, worktree_git_path(wt, "index")) > 0) {
+		for (i = 0; i < istate.cache_nr; i++) {
+			struct cache_entry *ce = istate.cache[i];
+
+			if (S_ISGITLINK(ce->ce_mode)) {
+				found_submodules = 1;
+				break;
+			}
+		}
+	}
+	discard_index(&istate);
+
+	if (found_submodules)
+		die(_("working trees containing submodules cannot be moved"));
+}
+
 static int move_worktree(int ac, const char **av, const char *prefix)
 {
 	struct option options[] = {
@@ -643,6 +664,8 @@ static int move_worktree(int ac, const char **av, const char *prefix)
 	if (file_exists(dst.buf))
 		die(_("target '%s' already exists"), dst.buf);
 
+	validate_no_submodules(wt);
+
 	reason = is_worktree_locked(wt);
 	if (reason) {
 		if (*reason)