Fix UnboundLocalError when posting Perforce paths.

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`.

Testing Done:
- 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.

Reviewed at https://reviews.reviewboard.org/r/5834/