rbt land --squash was correctly squashing commits when landing a local

branch, but not when loading a review request with multiple commits. All

the work to handle squashing a patch was already done, but we just

weren't plumbing the --squash argument to rbt patch to trigger it.
This is a simple change that adds the missing argument, fixing squashing

of multi-commit review requests.

Successfully landed a multi-commit review request with --squash.