Merge using specific commit instead of branch name

[OddBloke]

When I'm working on a complex feature, I will often submit a part of the work as a merge proposal whilst continuing to work locally (in the same branch). When lpshipit performs a merge, it uses the branch name not the commit that was actually submitted for review, which means it merges my local changes in addition to those that have been reviewed. It should, instead, explicitly merge the commit that was proposed.