[MERGE] Add scan_unvalidated_index to _KnitGraphIndex, add external_refernces to GraphIndex and BTreeGraphIndex.

[Andrew Bennetts]

[I tried to send this yesterday, but screwed up my From address, and couldn't
find it in the moderation queue.]

This patch adds a new API, scan_unvalidated_index, to _KnitGraphIndex.  This is
intended for use by resume_write_group, so that a _KnitGraphIndex can
reconstruct the list of missing compression parents that insert_record_stream
would have built up if those records had been added in a single unresumed write
group.

This partly replaces the patch that John voted bb:resubmit for, by keeping the
state about missing compression parents in the _KnitGraphIndex rather than the
KnitVersionedFiles.

It also adds an external_references method to GraphIndex/BTreeGraphIndex.

-Andrew.