ci: github action - add check for whitespace errors

ci: github action - add check for whitespace errors

Not all developers are aware of `git diff --check` to warn
about whitespace issues.  Running a check when a pull request is
opened or updated can save time for reviewers and the submitter.

A GitHub workflow will run when a pull request is created or the
contents are updated to check the patch series.  A pull request
provides the necessary information (number of commits) to only
check the patch series.

To ensure the developer is aware of any issues, a comment will be
added to the pull request with the check errors.

Signed-off-by: Chris. Webster <chris@webstech.net>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>