t / t4135 / diff-with tab.diffon commit sha1_name.c: get_describe_name() by definition groks only commits (6269b6b)
   1--- post        image.txt.orig  2010-08-18 20:13:31.616002255 -0500
   2+++ post        image.txt       2010-08-18 20:13:31.616002255 -0500
   3@@ -1 +1 @@
   4-preimage
   5+postimage