A pruned commit just indicates that its predecessors should be evolved
onto the pruned commit's parent instead of onto the pruned commit
itself. The pruned commit itself can be divergent. For example, if
there are several pruned sucessors of a commit, then it's unclear
where the predecessor's children should be rebased to.
Before this commit, when the `evolve()` evolved a stack of orphans, it
would use the evolve state from the beginning of the function to
calculate where they should go. That meant that only the bottom-most
orphan(s) would get evolved to their right place. This commit fixes
that by use the Transaction's evolution state.
