.arcconfig

git: add test showing `hg commit -i` working on a git repo

Shows `hg commit -i` working on a git repo via the `git`
extension. Adds working directory changes to files `alpha` and
`beta`, then selects only `alpha` changes and commits them.

As of now this would fail for a filename that
includes uppercase characters due to the lack of case folding
support in the extension.

Differential Revision: https://phab.mercurial-scm.org/D9081

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