Summary: Ref T13546. Ref T9948. It seems challenging to examine a remote in vanilla Mercurial. Provide an "hg arc-ls-remote" command which functions like "git ls-remote" so we can figure out if "--into X" is a bookmark, branch, both, neither, or a branch with multiple heads without mutating the working copy as a side effect.
Test Plan: Ran various "arc land --into ..." commands in a Mercurial working copy, saw apparently-sensible resolution of remote marker names.
Maniphest Tasks: T13546, T9948
Differential Revision: https://secure.phabricator.com/D21343
