Fix rbt land --squash when landing a review request.

`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.

Testing Done:
Successfully landed a multi-commit review request with `--squash`.

Reviewed at https://reviews.reviewboard.org/r/11296/

rbtools/commands/land.py

@@ -119,8 +119,20 @@ class Land(Command):
         Command.branch_options,
     ]
 
-    def patch(self, review_request_id):
-        """Patch a single review request's diff using rbt patch."""
+    def patch(self, review_request_id, squash=False):
+        """Patch a single review request's diff using rbt patch.
+
+        Args:
+            review_request_id (int):
+                The ID of the review request to patch.
+
+            squash (bool, optional):
+                Whether to squash multiple commits into a single commit.
+
+        Raises:
+            rbtools.commands.CommandError:
+                There was an error applying the patch.
+        """
         patch_command = [RB_MAIN, 'patch']
         patch_command.extend(build_rbtools_cmd_argv(self.options))
 
@@ -129,6 +141,9 @@ def patch(self, review_request_id):
         else:
             patch_command.append('-C')
 
+        if squash:
+            patch_command.append('--squash')
+
         patch_command.append(six.text_type(review_request_id))
 
         rc, output = execute(patch_command, ignore_errors=True,
@@ -230,7 +245,8 @@ def land(self, destination_branch, review_request, source_branch=None,
             print('Applying patch from review request %s.' % review_request.id)
 
             if not dry_run:
-                self.patch(review_request.id)
+                self.patch(review_request.id,
+                           squash=squash)
 
         print('Review request %s has landed on "%s".' %
               (review_request.id, self.options.destination_branch))