cache: create `cache` directory at init time
The cache directory will be needed very quickly, so it seems simpler to create
it early to make sure it has the same owner and permission than the other
directory in the repository.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}