'bzr uncommit' of merge could use parent_tree from the dirstate

Somewhat related to bug #780544 (because both are about uncommit performance).

At the moment, we cache merge parents in the dirstate file (if you do 'bzr merge foo' and then 'bzr uncommit', the final tree would have 'foo' as one of the parents in the uncommitted revision). We could use the cached version to populate the new dirstate file.

However, the use case is pretty rare. While it may be common to uncommit past a merge (causing us to want to include that in the final dirstate), it is probably uncommit to merge then uncommit. So the priority on this is low, just something to keep in mind.