Add .arcconfig file to point arcanist to the KDE Phabricator instance.

Summary:
Arcanist is the commandline tooling (client) for Phabricator.
Phabricator is to replace the KDE reviewboard and become the new central hub in KDE workflow.

The .arcconfig points arcanist to the KDE Phabricator instance.
Additionally, to play it safe the config forbids arcanist from silently rewriting git history.

Reviewers: #kde_games, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, #kde_games

Differential Revision: https://phabricator.kde.org/D5545