sync: consider linking to prob-specs commits

[ee7]

When syncing, a track maintainer will often want to look at the recent changes to prob-specs to see why e.g. a test case was added or reimplemented.

Configlet could make that easier and faster by linking to the relevant commits. Now that we cache the prob-specs repo, and don't support a shallow clone of it (which we did when we had --prob-specs-dir), configlet has access to the entire problem-specifications repo history. So we could consider outputting a list of recent commits that touch e.g. a relevant canonical-data.json file.

Not a high priority for now.