[MERGE] Tolerate ghosts in mainline (#235055)

[Aaron Bentley]

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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.

Aaron
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