Improve scc unrebased-prs output

This PR:
- decreases the verbosity of the command: the header is only displayed if unrebased PRs are found between `branch1` and `branch2`
- implements submodule processing in `scc unrebased-prs`
- exist with a return code corresponding to the number of unrebased/mismatching PRs if this number is non zero.

Additionally, integration tests are refactored to test this return code and the submodule processing (using the `--shallow` option).