[MERGE] get_revision_graph_with_ghosts and fix for common_ancestor.

[Robert Collins]

This is a smallish patch to correct the common_ancestor behaviour in the
case of ghosts available on other branches, and introduces the discussed
get_revision_graph_ghost api.

This is further optimisable by having the MultipleRevisionSources api
acccept multiple revisions for its graph queries so that a single call
into it returns the combined data but I figured we can defer that until
we see what the raw knit performance is like.

Rob
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