If a user has a bookmark named 'arcpatch' in their mercurial repository already, 'arc patch D1234' fails to find a valid name. This is likely due to the lack of escaping in ArcanistPatchWorkflow::getBookmarkName (https://secure.phabricator.com/diffusion/ARC/browse/master/src/workflow/ArcanistPatchWorkflow.php$267) and mercurial sees arcpatch-D1234 as a query instead of a name. Using phabricator's hgsprintf to create the query should fix it.