t / t4101 / diff.0-3on commit sha1_name.c: teach get_short_sha1() a commit-only option (aa1dec9)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,2 +1,3 @@
   4 a
   5-b
   6+c
   7+b
   8\ No newline at end of file