Merge branch 'jt/push-negotiation-fixes' into maint

Bugfix for common ancestor negotiation recently introduced in "git
push" codepath.

* jt/push-negotiation-fixes:
  fetch: die on invalid --negotiation-tip hash
  send-pack: fix push nego. when remote has refs
  send-pack: fix push.negotiate with remote helper

diff --cc builtin/fetch.c
Simple merge

diff --cc send-pack.c
Simple merge