[MERGE] Add a global branch write lock to cmd_uncommit and avoid unnecessary Branch.revno calls

[John Arbash Meinel]

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Robert Collins wrote:
...

>>
>> (Also, in theory we could lock_read() if we are in --dry-run as an added
>> safety precaution, but I'm not sure of the specific merits of that.)
> 
> Isn't tree optional?
> 
> -Rob

It probably is, but if it is available it should be locked.

John
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