[MERGE] Add RepositoryFormat.fast_deltas to signal fast delta creation.

[John Arbash Meinel]

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Robert Collins wrote:
> Signs so far are that the new commit code path is only faster on chk
> repositories (or others that can efficiently determine the changes in
> merged branches). So this adds an attribute to signal that.
> 
> -Rob
> 

BB:approve

John
=:->

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