scc rebase: bug fix and improvements

This PR mostly improves the `scc rebase` command:
- as noticed by @dpwrussell, `scc rebase --remote REMOTE` was not working correctly. Commit 04561c2 should fix this behaviour by reusing the `GitRepoCommand` logic.
- a corresponding `testRemote` integration test was added to validate the option
- if a remote named like the connected Github user exists, `scc rebase` will try to use this remote while pushing the rebased branch first and then defaults to the URL if failing
- additional checks are added to the `TestRebase.py` integration tests