patchbomb: use get_unique_pull_path
The patchbomb code does not support multiple destinations yet. It would not be
too hard to implemented since hg outgoing does. However that is a bit too
much of a detour right now. In addition that kind of "select outgoing
changesets for processing" code exist for various commands so the best would be
to factor it out later.
Let us use the dedicated API until this is solved.
This comes with a small output change in the test that I find more accurate. So
I kept it.
Differential Revision: https://phab.mercurial-scm.org/D10405