Code review comment for lp:~jameinel/bzr/2.0rc2-419241-merge-sort

This fixes a bug in KnownGraph.merge_sort() when there is a ghost as a mainline parent.

The old code didn't notice because we always pruned ghosts when passing in the parent_map, but it seems nicer to have merge_sort handle them directly. (Means fewer calls to _strip_NULL_ghosts :).

This is part of a critical regression right now for stuff like 'bzr log' on branches with mainline ghosts.

The second (more critical) part is coming soon.

(The code is built on the 2.0 release branch so it can be merged there primarily.)