[MERGE] Repository has a new method ``has_revisions``

[Robert Collins]

So this adds Repository.has_revisions. While I considered has_revision a
LBYL problem, it turns out that the logic for 'find revisions to pull'
quite reasonably doesn't want to ask for 'all revisions' and filter what
crosses the wire :).

This provides a method which answers the question for many revisions, an
apparent necessity for smart server operations.

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