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).

Typical output of this function is
```
sebastien@jrs-macbookpro-25399:snoopys-sandbox (dev_4_4) $ python ~/code/ome/scc/scc/main.py unrebased-prs dev_4_4 dev_4_4~
Repository: openmicroscopy/snoopys-sandbox
****************************************************************************************************
PRs on dev_4_4 without note/comment for dev_4_4~
****************************************************************************************************
  # PR 57 joshmoore 'Add sandbox-2 as submodule'
Repository: openmicroscopy/snoopys-sandbox-2
****************************************************************************************************
PRs on dev_4_4 without note/comment for dev_4_4~
****************************************************************************************************
  # PR 2 sbesson 'Add new file'
Found 2 unrebased PR(s) and 0 mismatching PR(s)
```