Merge branch 'nd/diff-with-path-params'

Merge branch 'nd/diff-with-path-params'

A few options of "git diff" did not work well when the command was
run from a subdirectory.

* nd/diff-with-path-params:
  diff: make -O and --output work in subdirectory
  diff-no-index: do not take a redundant prefix argument