Allow ignoring commits before a set date in `repositories.yaml`

[Dexterp37]

Sometimes, when adding Glean to new products, there's a transition phase in which the registry files are wrong or broken. The probe-scraper has a way to deal with that by specifying which commit date to start considering commits from.

Given how often we hit this problem with the application, it might be useful to have this as an option in repositories.yaml.