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

[Andrew Bennetts]

Robert Collins wrote:
> 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.

This looks good to me.

bb:approve

-Andrew.