[MERGE] updated deprecated of find_previous_heads

[Robert Collins]

Hi Aaron, I hope this address's your feedback.

I have:
 - removed the kind special casing so that we can correctly use the
revision level graph.
 - fixed upgrade to use the new api (I missed this before)
 - added hopefully comprehensive tests for the last modified revision
value and the per-file graph creation.

I have not:
 - made the ordering of per-file graph parents deterministic. I'll look
at this as the commit refactoring progresses.

Cheers,
Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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