accept Pull-Request without merging (BB-8301)

I use the evolve extension to have a proper history in my projects (eg https://bitbucket.org/logilab/pylint/), whose development may be handled on bitbucket.

So there are many cases where I want to accept a PR but don't want the merge proposed by bitbucket (slight modification done before integration, want only one changeset for the feature instead of a whole branch merged, etc).

I currently have to reject the PR telling it has actually been integrated, which is imo really not satisfying, at least from the submitter POV.

Until evolve workflow is properly supported, it would be worth adding a no-merge-accept feature where I could tell that the PR has been accepted and integrated in changeset 1a2b3c4d.