Improve unrebased PRs display logic (not displaying if empty)

Do not print unrebased PR header before computing all found PRs
Add logic to return net count of unrebased/mismatching PRs
Fix visual indent
Fix count return with no mismatching PR
Add test for `scc unrebased-prs` exiting with non-zero unrebased PRs
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
Do not use context manage to preserver Python 2.6 compatibility

Test `scc unrebased-prs` return code using a try/except block
Add failing test for mismatching PRs check
Fix mismatching PRs count
Fix integration tests to test mismatch PRs check and --no-check option
Merge pull request #108 from sbesson/103_unrebased_output

Improve scc unrebased-prs output