t / t4135 / diff-with backslash.diffon commit use strbuf_add_unique_abbrev() for adding short hashes (1eb47f1)
   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