Breaks down the cases in which "git push" fails due to non-ff into three
categories, and gives separate advise messages. This should be a good
change regardless of mm/push-default-switch-warning topic.
Kicked back to 'pu'.
Needs adjusting the existing tests.
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