t / t4101 / diff.3-0on commit sha1_name: support @{-N} syntax in get_sha1() (d18ba22)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,2 @@
   4 a
   5-c
   6-b
   7\ No newline at end of file
   8+b