[merge] change branchbuilder example to a doctest

[John Arbash Meinel]

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Martin Pool wrote:
> Branchbuilder's docstring contains an example of how to use it, but
> it's not actually correct.  This changes it to a docstring so it can
> be tested, changes branchbuilder not to emit messages to the user when
> it commits, and fixes the docstring to actually work. ;-)
> 
> doctests are not great for general testing but it is nice to make sure
> api examples work...
> 

BB:approve

John
=:->

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