Target branch of the new Merge Request suggestion

This is related to #138 (closed), but has apparently never been filed.

The Merge Request creation suggested link that one gets upon pushes always relates to the default (GitLab) branch, but that's not expected for Mercurial topics, whose changesets already contain the logical target branch.

For a typical example, assuming the default GitLab branch to be branch/default: If I have a stable branch and I push changesets for that branch with the topic bugfix, hence creating a topic/stable/bugfix GitLab branch, but the MR suggestion link would be to merge into branch/default instead of the expected branch/stable.

On a project such as Heptapod itself, where the difference between a stable branch and the default is often several thousands of changesets, this stresses the web application with a huge diff. The target branch can be corrected afterwards, but that's painful for no good reason. In some cases, it can lead to unwanted merges, manual grafts or backports etc.