builtin-add: fix unmatched pathspec warnings.

"git add Documentation/" when Documentation directory exists
does not barf (as it should not), but "git add ." barfed when it
did not add anything.  This was because we checked for the path
prefix ("Documentation/" in the former case, and an empty string
in the latter case) for existence, and lstat("", &st) would say
"Huh?".

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

builtin-add.c

@@ -123,7 +123,7 @@ static void prune_directory(struct dir_struct *dir, const char **pathspec, int p
 
 		/* Existing file? We must have ignored it */
 		match = pathspec[i];
-		if (!lstat(match, &st))
+		if (!match[0] || !lstat(match, &st))
 			continue;
 		die("pathspec '%s' did not match any files", match);
 	}