[MERGE] repository write groups

[Aaron Bentley]

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Robert Collins wrote:
>> It isn't quite clear to me who gets to decide when to start the write 
>> group, since now you are having the transaction start/end be 
>> non-reentrant. Are you trying to make low-level functions just assert 
>> that we are in a write group, and high-level ones must take a write 
>> group?
> 
> these are the mutators of repositories that I know of:
>  - fetch

Also, bundle installation.

Aaron
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