pull request: use unionrepo instead of outgoing

This makes it possible to look the 'moving target' symbols up in the right repo.

Using a revset with the right revisions also removes the need for pruning
changesets that are outside the requested range.

It will also not be confused by changesets that for some reason has been pulled
to the repo but haven't been merged yet. They are going to be 'merged' by the
'pull' request and should thus be a part of what is reviewed.