.arcconfig

changelog: add a `changes` property on `changelogrevision`

For the sidedata storage we are moving toward "all in one" block containing the
equivalent of a "ChangingFiles" instance. We do various refactoring beforehand
to prepare the usage of theses new data in the code.

Since the object use slots, the "property cache" tricks cannot be used, and we
cache the value manually.

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