clone.py should handle case 'local branch with no remote'

The command ./clone.py someBranch -f fallback does the following:

• if someBranch is a tag, checkout it
• else if someBranch does not exist locally
  • if someBranch exists remotely, checkout and track it
  • else checkout fallback (recursive call to the same logic)
• else checkout local someBranch and rebase with its remote

This will fail if someBranch was created locally and not yet pushed.
=> rebase only if the remote exists