settling dirstate-tracked-key in 6.1

[Pierre-Yves David]

Hello everyone

The 6.1 freeze is looming and I would like to discuss finalizing the 
`dirstate-tracked-key` feature.
The main remaining question if the configuration option. That already 
sparked some discussion in the initial series but was a secondary issue 
as long as the feature experimental.

So after thinking more about this I don't think the `version` concerns 
warrant and special case. And I advocate for the usual 
`format.use-dirstate-tracked-key=yes` option. If needed, the version 
configuration might be dealt with by a secondary option: probably 
`format.use-dirstate-tracked-key:version`. It might be smart to 
introduce it already, to properly bark when the value is not "1". 
However this might seems premature.

What do you think ?

Cheers

PS: I am adding Kyle in CC as this feature is the related to a need he 
expressed with dirstate and concurrency issue Google had. This is not 
exactly what they wanted, but close enough to make sure they are aware 
of it. It might even fit there underlying needs ? I though I did the CC 
on the initial series but I apparently did not.

-- 
Pierre-Yves David