Use the current head as the default commit in diffs_metadata.json
When the merge request page requests diffs_metadata.json
the commit
property is only populated if the frontend also sends along the commit_id
with the request.
When the frontend doesn't send an ID, the resulting metadata is for the current head
of the branch for the Merge Request in question. The commit
in the response should reflect that default by returning the head_sha
.