Merge branch 'jc/diff-no-index-in-subdir' into next

"git diff --no-index" run inside a subdirectory under control of a
Git repository operated at the top of the working tree and stripped
the prefix from the output, and oddballs like "-" (stdin) did not
work correctly because of it.  Correct the set-up by undoing what
the set-up sequence did to cwd and prefix.

* jc/diff-no-index-in-subdir:
  diff: --no-index should ignore the worktree