Clean t/trash upon "make clean" as well.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/Makefile b/Makefile
index 4a38aa6..dfb8c0a 100644
--- a/Makefile
+++ b/Makefile
@@ -235,3 +235,4 @@
 	rm -f git-core-*.tar.gz git-core.spec
 	$(MAKE) -C tools/ clean
 	$(MAKE) -C Documentation/ clean
+	$(MAKE) -C t/ clean