tree_entry_interesting(): support depth limit

This is needed to replace pathspec_matches() in builtin/grep.c.

max_depth == -1 means infinite depth. Depth limit is only effective
when pathspec.recursive == 1. When pathspec.recursive == 0, the
behavior depends on match functions: non-recursive for
tree_entry_interesting() and recursive for match_pathspec{,_depth}

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

dir.c

@@ -87,6 +87,21 @@ int fill_directory(struct dir_struct *dir, const char **pathspec)
 	return len;
 }
 
+int within_depth(const char *name, int namelen,
+			int depth, int max_depth)
+{
+	const char *cp = name, *cpe = name + namelen;
+
+	while (cp < cpe) {
+		if (*cp++ != '/')
+			continue;
+		depth++;
+		if (depth > max_depth)
+			return 0;
+	}
+	return 1;
+}
+
 /*
  * Does 'match' match the given name?
  * A match is found if