.arcconfig

exthelper: reintroduce the ability to register templates

Same mechanism as revsets earlier in this series.  The LFS extension is updated
to provide test coverage.

This also seems to make the test failure around issue6033 mentioned in
0a7f582f6f1f much less regular on Windows.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}