dateformat: parse %(xxdate) %(yydate:format) correctly

Andy Parkins noticed that parsing of the above would not
correctly notice that xxdate does not have any format
specifier.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-for-each-ref.c

@@ -113,7 +113,7 @@ static int parse_atom(const char *atom, const char *ep)
 		 * table.
 		 */
 		const char *formatp = strchr(sp, ':');
-		if (!formatp)
+		if (!formatp || ep < formatp)
 			formatp = ep;
 		if (len == formatp - sp && !memcmp(valid_atom[i].name, sp, len))
 			break;