[MERGE] Make log revision filtering pluggable.

[Robert Collins]

So this is a little bit of an overhaul of how log works.

It:
 - makes log -v -m foo much faster, by not generating deltas until we're
sure we will show a revision
 - it makes the separate steps (batching, reading revisions, making
deltas, and doing regex search evaluation) separate filter functions.
 - it allows plugins to control the separate steps in a fairly crude
way.

-Rob

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