.arcconfig

commitextras: work nicely with other extensions

Before this change, it doesn't add these extra fields when loaded
alongside another extension that does a bunch of things, including
wrapping commit.

I did not investigate exactly why, but
- the documentation of extensions.wrapfunction says to use subclassing
to play nicely with other extensions
- using subclassing does make commitextras work when loaded alongside
my other extension

Differential Revision: https://phab.mercurial-scm.org/D4404

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