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

support/hg/arc-hg.py

@@ -159,21 +159,16 @@ def remotemarkers(ui, repo, source, opts):
   source, branches = hg.parseurl(ui.expandpath(source))
   remote = hg.peer(repo, opts, source)
 
-  bundle, remotebranches, cleanup = bundlerepo.getremotechanges(
-    ui,
-    repo,
-    remote)
+  with remote.commandexecutor() as e:
+    branchmap = e.callcommand('branchmap', {}).result()
 
-  try:
-    for n in remotebranches:
-      ctx = bundle[n]
+  for branch_name in branchmap:
+    for branch_node in branchmap[branch_name]:
       markers.append({
         'type': 'branch',
-        'name': ctx.branch(),
-        'node': node.hex(ctx.node()),
+        'name': branch_name,
+        'node': node.hex(branch_node),
       })
-  finally:
-    cleanup()
 
   with remote.commandexecutor() as e:
     remotemarks = bookmarks.unhexlifybookmarks(e.callcommand('listkeys', {