Break down the cases in which "git push" fails due to non-ff into
three categories, and give separate advise messages for each case.

By Christopher Tiwald (2) and Jeff King (1)
* ct/advise-push-default:
  Fix httpd tests that broke when non-ff push advice changed
  clean up struct ref's nonfastforward field
  push: Provide situational hints for non-fast-forward errors