untracked cache: invalidate dirs recursively if .gitignore changes

It's easy to see that if an existing .gitignore changes, its SHA-1
would be different and invalidate_gitignore() is called.

If .gitignore is removed, add_excludes() will treat it like an empty
.gitignore, which again should invalidate the cached directory data.

if .gitignore is added, lookup_untracked() already fills initial
.gitignore SHA-1 as "empty file", so again invalidate_gitignore() is
called.

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

dir.c

@@ -1011,7 +1011,23 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 			add_excludes(el->src, el->src, stk->baselen, el, 1,
 				     untracked ? &sha1_stat : NULL);
 		}
-		if (untracked) {
+		/*
+		 * NEEDSWORK: when untracked cache is enabled, prep_exclude()
+		 * will first be called in valid_cached_dir() then maybe many
+		 * times more in last_exclude_matching(). When the cache is
+		 * used, last_exclude_matching() will not be called and
+		 * reading .gitignore content will be a waste.
+		 *
+		 * So when it's called by valid_cached_dir() and we can get
+		 * .gitignore SHA-1 from the index (i.e. .gitignore is not
+		 * modified on work tree), we could delay reading the
+		 * .gitignore content until we absolutely need it in
+		 * last_exclude_matching(). Be careful about ignore rule
+		 * order, though, if you do that.
+		 */
+		if (untracked &&
+		    hashcmp(sha1_stat.sha1, untracked->exclude_sha1)) {
+			invalidate_gitignore(dir->untracked, untracked);
 			hashcpy(untracked->exclude_sha1, sha1_stat.sha1);
 		}
 		dir->exclude_stack = stk;