[MERGE] Fix a bug with how PendingAncestryResult.get_keys handles NULL_REVISION.

[Andrew Bennetts]

This fixes a bug that affected Vincent.  If graph.iter_ancestry(key) returns
NULL_REVISION somewhere other than at the end of the history (as can occur
when there are non-lefthand root revisions), then PendingAncestryResult
would stupidly raise an AssertionError.

What we actually care about is filtering out NULL_REVISION whereever it does
occur (because we don't want to try to fetch records for it), so this patch
does that rather than expecting it to be the final key in the ancestry.

-Andrew.

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