Fix str(None) bug in port detection for hook setup

1) when git-review -s is launched it tries to detect
   information for scp-ing the commit-msg hook from remote.
2) when remote url does not contains port (':number')
   we'll get None from urlparse.
3) just before passing it further it was always converted
   to string (and so we could get 'None')
4) Later if port is not None, we add '-P {port}' to the scp command.

Which caused scp failure with message 'Bad port None'.

So we convert port to str only if it's not None.

Change-Id: I64c7a197844027cb24c13817d6c3186009568e8a