Use a "branchmap" call to identify remote branches in "arc-hg"

Summary:
Ref T9948. Ref T13546. To identify remote branch heads -- not just modified heads -- use "branchmap" like "hg outgoing" does.

I wasn't able to find any other way to get what we want: for example, with a bundlerepo, "peer.heads()" and "peer.changelog.heads()" include local branches which are not present in the remote.

It generally seems difficult (perhaps impossible?) to distinguish between these cases by using "getremotechanges()":

• branch X exists at position Y in both the local and remote;
• branch X exists at positino Y in the local, but not the remote.

In any case, this seems to work properly and should do less work than "getremotechanges()" since we don't need to pull as much data over the wire.

Test Plan: Ran "hg arc-ls-markers" in various repositories, got what appeared to be a faithful representation of the remote branch and bookmark state.

Maniphest Tasks: T13546, T9948

Differential Revision: https://secure.phabricator.com/D21349