t / t4135 / add-plain.diffon commit dir: convert read_skip_worktree_file_from_index to take an index (6f52b74)
   1diff -pruN a/postimage.txt b/postimage.txt
   2--- a/postimage.txt     1969-12-31 18:00:00.000000000 -0600
   3+++ b/postimage.txt     2010-08-18 20:13:31.484002255 -0500
   4@@ -0,0 +1 @@
   5+postimage