[API] Provide the merge-commit hash after a succesfull merge

We have build a tool that works with multiple Git portals including GitLab but one thing that is currently missing for us the merge-commit hash after a successful merge http://docs.gitlab.com/ce/api/merge_requests.html#accept-mr

We use this information for storing git-notes with all comments, but to do this we need the merge-commit.

Is it possible to expose this? I looked at the source code but I know nothing about Ruby to create patch 😇