Pull requests should go back to forked-from repo by default (BB-9378)

I'm setting up a workflow where each developer has his own fork on Bitbucket. He pushes his changesets there and issue a pull request for someone to review the changesets before merging them back into the main repository.

It would therefore be helpful if the pull request screen would default to the main repo as the pull request destination. The named branch should also be kept as is: if I make a change on the stable branch in my repo, I want the change to be on the stable branch in the main repo too.

Generally, I think it makes little sense to issue cross-named branch pull requests with Mercurial. For Git or when using Mercurial bookmarks it makes a lot of sense, but named branches are meant for long-term branches where you don't merge on a per-feature basis.