t / t4101 / diff.1-2on commit append_one_rev(): rewrite to work with object_id (7a456c1)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,2 @@
   4 a
   5-b
   6-c
   7+b
   8\ No newline at end of file