t / t4101 / diff.1-2on commit dir: convert read_skip_worktree_file_from_index to take an index (6f52b74)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,2 @@
   4 a
   5-b
   6-c
   7+b
   8\ No newline at end of file