untracked cache: invalidate at index addition or removal

Ideally we should implement untracked_cache_remove_from_index() and
untracked_cache_add_to_index() so that they update untracked cache
right away instead of invalidating it and wait for read_directory()
next time to deal with it. But that may need some more work in
unpack-trees.c. So stay simple as the first step.

The new call in add_index_entry_with_check() may look strange because
new calls usually stay close to cache_tree_invalidate_path(). We do it
a bit later than c_t_i_p() in this function because if it's about
replacing the entry with the same name, we don't care (but cache-tree
does).

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

dir.c

@@ -2502,3 +2502,34 @@ done2:
 	}
 	return uc;
 }
+
+void untracked_cache_invalidate_path(struct index_state *istate,
+				     const char *path)
+{
+	const char *sep;
+	struct untracked_cache_dir *d;
+	if (!istate->untracked || !istate->untracked->root)
+		return;
+	sep = strrchr(path, '/');
+	if (sep)
+		d = lookup_untracked(istate->untracked,
+				     istate->untracked->root,
+				     path, sep - path);
+	else
+		d = istate->untracked->root;
+	istate->untracked->dir_invalidated++;
+	d->valid = 0;
+	d->untracked_nr = 0;
+}
+
+void untracked_cache_remove_from_index(struct index_state *istate,
+				       const char *path)
+{
+	untracked_cache_invalidate_path(istate, path);
+}
+
+void untracked_cache_add_to_index(struct index_state *istate,
+				  const char *path)
+{
+	untracked_cache_invalidate_path(istate, path);
+}