t / t4101 / diff.2-1on commit t/t7003-filter-branch.sh: use test_must_fail rather than '!' (c8a0869)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,2 +1,3 @@
   4 a
   5-b
   6\ No newline at end of file
   7+b
   8+c