Merge branch 'jc/diff-no-index-d-f' into maint-2.3

The usual "git diff" when seeing a file turning into a directory
showed a patchset to remove the file and create all files in the
directory, but "git diff --no-index" simply refused to work.  Also,
when asked to compare a file and a directory, imitate POSIX "diff"
and compare the file with the file with the same name in the
directory, instead of refusing to run.

* jc/diff-no-index-d-f:
  diff-no-index: align D/F handling with that of normal Git
  diff-no-index: DWIM "diff D F" into "diff D/F F"