replace: avoid using die() to indicate a bug

We have the BUG() macro for that purpose.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/replace.c

@@ -501,6 +501,6 @@ int cmd_replace(int argc, const char **argv, const char *prefix)
 		return list_replace_refs(argv[0], format);
 
 	default:
-		die("BUG: invalid cmdmode %d", (int)cmdmode);
+		BUG("invalid cmdmode %d", (int)cmdmode);
 	}
 }