Discussion lost when merge request closed, merged manually and reopened

Summary

There is no history/discussion in merge request from fork when commits count is zero. Not sure why it is zero sometimes, nonetheless we should at least preserve discussion.

Steps to reproduce

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6240

Expected behavior

We should display some relevant information under the merge request widget.

Actual behavior

There is nothing below the merge request widget.

/cc @DouweM