[land-bot] Split arc land into distinct parts in preparation for supporting non-revision patches

Summary:
Apparently Phabricator will still close revisions when git push is used instead of arc land.
This is good news, as it will allow us to reuse more of this code than previously thought for landing
automated commits.

There is no behavior change.

Test Plan: CONDUIT_TOKEN=<token> ./land-patch.sh --dry-run Dxxxx

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7455