Posting Perforce paths using rbt post would sometimes result in an

UnboundLocalError because the local variable new_path was referenced before

assignment. This is a trivial fix, changing the argument from new_path to

old_path.

Ran unit tests.
Ran rbt post //p4-test2/foo@2,@3 (which previously resulted in the error),

  and saw that the review request was created successfully.