t/Makefile: OPTS -> GIT_TEST_OPTS

This way, passing this variable through the environment actually makes
sense.

t/Makefile

@@ -2,12 +2,13 @@
 #
 # Copyright (c) 2005 Junio C Hamano
 #
-#OPTS=--verbose --debug
+
+#GIT_TEST_OPTS=--verbose --debug
 
 T = $(wildcard t[0-9][0-9][0-9][0-9]-*.sh)
 
 all:
-	@$(foreach t,$T,echo "*** $t ***"; sh $t $(OPTS) || exit; )
+	@$(foreach t,$T,echo "*** $t ***"; sh $t $(GIT_TEST_OPTS) || exit; )
 	@rm -fr trash
 
 clean: