[RFC] remove versioned_file.enable_cache

[John Arbash Meinel]

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Robert Collins wrote:
> the enable_cache method seems to be only called once, for knit repositor
> fetching. I think we can safely remove the cache logic entirely as packs
> are the default. (That or make the cache be a decorator/optional layer).
> 
> -Rob

Specifically, it was used because we download inventories first, to
figure out what texts we need to process. But we can't install the
inventories in a Knit repository.

If you remove this completely, it will significantly hurt Knit format
fetching. Because it has to download the inventories 2x.

So if you want to refactor it so that it is only enabled for Inventories
when fetching with Knit format repositories, I'm okay with that. But I
don't think we want such a significant regression on everyone who hasn't
upgraded yet.

John
=:->
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