I just found out that arc browse takes a --branch argument, which is super awesome. When I use it to open a path in a branch that contains a slash, however (e.g. releases/42), the URI that Arcanist opens doesn't percent encode the branch name in the URL (e.g. https://phabricator.evand.org/diffusion/X/browse/releases/42/AppDelegate.m), so Diffusion throws an exception (e.g. Ref "releases" does not exist in this repository).