Prevent post-receive error when push to project with dead forks

If project has open merge request from fork and this fork was removed
before merge request was closed it cause exception during push

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Refactor merge request refresh logic on push

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'fix-deleted-fork' into 'master'

Prevent post-receive error when push to project with dead forks

Fixes gitlab/gitlab-cloud#173

See merge request !1246