Record `wiby test` state

[dominykas]

Ref: https://github.com/pkgjs/wiby/blob/master/docs/DRAFT-FLOW.md step 3, step 10

wiby will look for an existing log file for this HEAD, and error if it finds one. This will stop wiby test being run multiple times. You can override this check with wiby test --overwrite.

We might want to record the test references, e.g. which repos actually got kicked off, which commit status to check, etc, as at the moment it just relies on a matching branch in dependent repos.

Some options where to put that data:

• a log file, as suggested initially
  • only available locally, but maybe that's good enough?
• commit status
  • unsure how much metadata we can store there
  • a variation would be to have a status per dependent, but unsure if there are any limits on how many status checks can a commit have
• git notes
  • might be trickier to implement and would need some dancing around
• just stick to a convention and re-scan all the dependents based on wiby.json each time