tests: flatten repo structure in test-commit.t
Let's move to parent directory before `hg init` repos, since they don't need to
be nested. It makes amend/strip messages that include full path to the backup
bundle shorter, for instance.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}