Add a config key cache_revs to specify how many revs to keep
Summary:
This should be less necessary after T137124: Scap3 submodule space issues is resolved but why not.
Fixes T181176
We <3 scap.cfg
Test Plan: didn't test it
Reviewers: thcipriani, demon, Release-Engineering-Team
Subscribers: awight
Tags: Release-Engineering-Team
Maniphest Tasks: T181176
Differential Revision: https://phabricator.wikimedia.org/D900