t / t4135 / diff-plain.diffon commit setup: set env $GIT_WORK_TREE when work tree is set, like $GIT_DIR (d95138e)
   1--- postimage.txt.orig  2010-08-18 20:13:31.432002255 -0500
   2+++ postimage.txt       2010-08-18 20:13:31.432002255 -0500
   3@@ -1 +1 @@
   4-preimage
   5+postimage