Hg-git should keep updating git branches linked to published topics

When push-publishing a freshly rebased topic, the git mirror doesn't move the corresponding branch: it is just deleted, while the target branch is updated to the tip. In this scenario, Gitlab fails to understand that the MR linked to the topic is now merged, and just considers that the source branch was deleted.

When not pruning remote branches, hggit should probably update the git branches corresponding to a published topic to their new position.