rbt 2.0 introduced the possibility to provide the
review request number as full URL. But this wasn't
possible for --print option.

  rbt patch --print https://reviewboard/r/12345
  ERROR: Unable to find a Review Board server for this source code tree.

This change allows the usage, too.

Tried rbt patch --print https://reviewboard/r/12345 after fix

and saw that is printed the diff to stdout.
Tried rbt patch --print 12345 without repository and saw

that the error from above.