Merge branch 'ct/advise-push-default'

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

diff --cc builtin/push.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc t/t5541-http-push.sh
Simple merge

diff --cc transport.c
Simple merge