t / t4135 / diff-with backslash.diffon commit dir: use the_hash_algo for empty blob object ID (ba2df75)
   1--- post\image.txt.orig 2010-08-18 20:13:31.680002255 -0500
   2+++ post\image.txt      2010-08-18 20:13:31.680002255 -0500
   3@@ -1 +1 @@
   4-preimage
   5+postimage