t6001: avoid direct file system access

Signed-off-by: Han-Wen Nienhuys <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: hanwen

t/t6001-rev-list-graft.sh

@@ -23,7 +23,8 @@ test_expect_success setup '
 	git commit -a -m "Third in one history." &&
 	A2=$(git rev-parse --verify HEAD) &&
 
-	rm -f .git/refs/heads/main .git/index &&
+	git update-ref -d refs/heads/main &&
+	rm -f .git/index &&
 
 	echo >fileA fileA again &&
 	echo >subdir/fileB fileB again &&