Improve content of Stop message in `scc unrebased-prs`

Display counts of unrebased PRs and mismatching PRs
Closes #103
Fix scc unrebased-prs to support submodule recursivity
Fix UnrebasedPRs integration test

- Use context manager to check exception return code for unrebased prs
- Test shalle vs non-shallow unrebased-prs using last first-parent commit