largefiles: update lfutil.findoutgoing() discovery method

It looks like this method missed the updates in 30273f0c776b (which changed the
preferred discovery method from findcommonincoming() to findcommonoutgoing()),
and cd956049fc14 (which rolls up the outgoing lists into a single object).

hgext/largefiles/lfutil.py

@@ -52,9 +52,8 b' def repoforget(repo, list):'
 
 def findoutgoing(repo, remote, force):
     from mercurial import discovery
-    common, _anyinc, _heads = discovery.findcommonincoming(repo,
-        remote.peer(), force=force)
-    return repo.changelog.findmissing(common)
+    outgoing = discovery.findcommonoutgoing(repo, remote.peer(), force=force)
+    return outgoing.missing
 
 # -- Private worker functions ------------------------------------------