[MERGE] add performance tweaks.

[Aaron Bentley]

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+1 with the changes you've described.

Robert Collins wrote:
>> I don't understand the difference between the top and the prefix.  Why
>> would I specify the top and then specify the prefix?  Is it so that the
>> path is still relative to top?
> 
> Yes, How about this docstring fragment:
> 
> :param prefix: Prefix the relpaths that are yielded with 'prefix'. This 
>   allows one to walk a subtree but get paths that are relative to a tree
>   rooted higher up.

I think this works.

Aaron
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