Teach --quiet to diff backends.

This teaches git-diff-files, git-diff-index and git-diff-tree
backends to exit early under --quiet option.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff-lib.c

@@ -324,6 +324,9 @@ int run_diff_files(struct rev_info *revs, int silent_on_removed)
 		struct cache_entry *ce = active_cache[i];
 		int changed;
 
+		if (revs->diffopt.quiet && revs->diffopt.has_changes)
+			break;
+
 		if (!ce_path_match(ce, revs->prune_data))
 			continue;
 
@@ -565,6 +568,9 @@ static int diff_cache(struct rev_info *revs,
 		struct cache_entry *ce = *ac;
 		int same = (entries > 1) && ce_same_name(ce, ac[1]);
 
+		if (revs->diffopt.quiet && revs->diffopt.has_changes)
+			break;
+
 		if (!ce_path_match(ce, pathspec))
 			goto skip_entry;
 

tree-diff.c

@@ -161,6 +161,8 @@ static void show_entry(struct diff_options *opt, const char *prefix, struct tree
 int diff_tree(struct tree_desc *t1, struct tree_desc *t2, const char *base, struct diff_options *opt)
 {
 	while (t1->size | t2->size) {
+		if (opt->quiet && opt->has_changes)
+			break;
 		if (opt->nr_paths && t1->size && !interesting(t1, base, opt)) {
 			update_tree_entry(t1);
 			continue;