Support pull-requests with automatic merging having pull-requests without automatic merging

Basically we're looking for a solution to this question: https://answers.atlassian.com/questions/13418077/with-stash-automatic-merge-how-can-i-avoid-an-automatic-merge at the pull-request level in Stash.

When doing a pull-request with automatic merging we want to say "do not auto-merge". The pull-request would then merge to the target branch normally but then do an "ours" merge down the "auto-merge cascade" so that subsequently the change would not be merged down.

Alternatively, potentially allow a cherry-pick/interactive merge for when there are commits that you may not want merged.