Diffusion Scap 70c0e1a5074a

Add a config key `cache_revs` to specify how many revs to keep


Add a config key cache_revs to specify how many revs to keep

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