add a spec for commit merge request diff notes

also, I refactored the MergeRequest#fetch_ref method to express
the side-effect that this method has.

  MergeRequest#fetch_ref -> MergeRequest#fetch_ref!
  Repository#fetch_source_branch -> Repository#fetch_source_branch!

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.

Removed merge_request/issue state_event param parent and removed issuable_path helpers in favour of issuable_url helpers