Merge branch 'ct/advise-push-default' into maint

The cases "git push" fails due to non-ff can be broken into three
categories; each case is given a separate advise message.

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 Documentation/config.txt
Simple merge

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