Pull Request Merge can result in multiple heads

My team recently started using BitBucket to host our source code and really like the server-side merge functionality included in pull requests. Unfortunately, this occasionally results in multiple heads being created on the destination branch after the merge. Most Mercurial usage patterns expect that a named branch only has a single head, to make it clear where the most up-to-date development is happening.

It would be fine if Bitbucket rejected the request and had the user bring the branch up to date first, but simply allowing the multiple heads is incorrect behavior by most Mercurial standards.

This is similar to BCLOUD-4334.