diff --cached: give more sensible error message when HEAD is yet to be created.

It is not like the user said 'diff --cached HEAD', so complaining about
HEAD not being a valid commit, while technically might be correct, is
not very helpful.

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

builtin-diff.c

@@ -261,6 +261,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 				break;
 			else if (!strcmp(arg, "--cached")) {
 				add_head(&rev);
+				if (!rev.pending.nr)
+					die("No HEAD commit to compare with (yet)");
 				break;
 			}
 		}