Put cache in .hg in the repository

[sheep]

Hatta needs a cache directory for storing some files. Currently it just creates a "cache" subdirectory wherever you tell it, and uses that -- by default in the directory where Hatta is started, often inside the repository. This is bad, as this directory should not be tracked.

Hatta could attempt to add the cache directory to .hgignore, but that's too much magic to my taste.

I think the best option is to find one good place for keeping the cache. The .hg directory of the repository sound like such a place. It's never tracked and Hatta has to locate it anyways. It also has to exist for Hatta to work, so we can remove one configuration option this way.

[sheep]

Hatta now defaults to .hg/hatta/cache for the cache directory, but it still can be changed with an option.

[sheep]

fixed in 614dad3d8a07