[land-bot] Extract revision status check code

Summary:
This piece can be more easily tested as a stand-alone script.
Extracting it will also make review a bit easier for some changes coming
down the pipe.

Test Plan: ninja check-source-control-tools

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7421