[MERGE] Detect versionedfile ancestries with unreferenced ancestors

[Aaron Bentley]

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Hi all,

This patch introduces Repository.find_bad_ancestors, which detects the
kind of corruption that prevents fileids_altered_by_revision_ids for
correctly determining which revisions are needed.

Say we have Revision 1.  Its parents are revision 2 and 3.  Its
inventory has a "revision" value of 1-t for a given file.

Say the "revision" value for that file in 2 is 2-t, and 3-t for 3.

The only legitimate versionedfile parent values for revision 1-t are 2-t
and 3-t.  The versionedfile may not indicate any other revisions as
parents for 1-t.

This is what my patch detects.  It does not attempt to detect other
problems because pragmatically, we need to fix this particular kind of
corruption soon, so we can make fetch more efficient.

Aaron
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