Allow passing extra arguments to rsync in sync_common()
What it says on the tin. This is designed so we can sometimes pass rsync args that we usually wouldn't otherwise want.
In this case? We want scap pull to optionally support --delete-excluded...a flag we absolutely would not want to otherwise use. That's T157030 for the viewers playing along at home.
Test Plan: It should work.
Reviewers: mmodell, Release-Engineering-Team, thcipriani
Reviewed By: Release-Engineering-Team, thcipriani
Differential Revision: https://phabricator.wikimedia.org/D981