Making chg stateful

[Jun Wu]

Fixes a mysterious typo that affects reading.

Excerpts from Jun Wu's message of 2017-02-02 09:34:47 +0000:
> This is mainly to discuss more details about chg repo preloading idea [1].
> 
> Perf Numbers
> 
>   I wrote a hacky prototype [2] that shows significant improvements on
>   various commands in our repo:
>   
>                                Before   After (in seconds)
>       chg bookmark             0.40     0.08
>       chg log -r . -T '{node}' 0.50     0.08
>       chg sl                   0.71     0.27
>       chg id                   0.71     0.37
>   
>   And hg-committed (with 12M obsstore) could benefit from it too mainly
>   because the obsstore becomes preloaded:
>   
>                                Before   After
>       chg bookmark             0.56     0.06
>       chg log -r . -T '{node}' 0.56     0.07
>       chg log -r . -p          0.86     0.08
>       chg sl                   0.84     0.21
>       chg id                   0.54     0.06
>   
>   So I think it's nice to get a formal implementation upstreamed. It's also
>   easier to solve the perf issues individually like building the hidden
           ^^^^^^^^ "to solve" should be "than solving"

>   bitmap, building an serialization format for the radix tree, etc.
>   Although we can also build them later.
>