Mark the wildcard argument to "arc liberate" as a path argument for shell completion

Summary: This is a path argument, and shell completion should suggest tabs.

Test Plan: Typed "arc liberate s<tab>", got path suggestions.

Differential Revision: https://secure.phabricator.com/D21292

src/workflow/ArcanistLiberateWorkflow.php

@@ -29,7 +29,8 @@ EOTEXT
         ->setHelp(
           pht('Perform a clean rebuild, ignoring caches. Thorough, but slow.')),
       $this->newWorkflowArgument('argv')
-        ->setWildcard(true),
+        ->setWildcard(true)
+        ->setIsPathArgument(true),
     );
   }