[bundle] sign-my-commits should use a write lock

[Aaron Bentley]

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John Arbash Meinel wrote:
> I just went to use 'bzr sign-my-commits' to make sure all the stuff I
> had worked on was signed, and it started to take forever. I looked at
> the code, and it wasn't taking a write-lock, so it wasn't caching
> anything in memory.
> 
> The attached patch should fix this behavior.

+1

Aaron
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