[MERGE] Tolerate ghosts in mainline (#235055)

[Andrew Bennetts]

Aaron Bentley wrote:
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> Hi all,
> 
> This patch fixes Branch6.revision_history so that it handles mainline
> ghosts the same as Branch5.
> 
> Specifically, ghosts are not permitted in any of the mainline revisions,
> but the lefthand parent of revision 1 may be a ghost.
> 
> The patch ensures this works, and has the same behavior in all branch
> implementations.

bb:approve

Also, this fixes "bzr branch bzr+ssh://..." from an affected branch.

-Andrew.