t / t4135 / diff-with tab.diffon commit check-ref-format doc: de-emphasize the implementation detail of a ref (a0a7e9e)
   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