Merge branch 'jk/diff-patch-dry-run-cleanup'

Finishing touches to fixes to the recent regression in "git diff -w
--quiet" and anything that needs to internally generate patch to
see if it turns empty.

* jk/diff-patch-dry-run-cleanup:
  diff: simplify run_external_diff() quiet logic
  diff: drop dry-run redirection to /dev/null
  diff: replace diff_options.dry_run flag with NULL file
  diff: drop save/restore of color_moved in dry-run mode
  diff: send external diff output to diff_options.file