pull -r: pass the revisions as the heads argument of findincoming
This can make a hg pull -r faster if the remote repo has many heads,
and fixes an "abort: received changelog group is empty".
adding a
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
a renamed from b:9a263dd772e0159ddfa70fb8a448bb6c7ec8c69f
% test with --rev
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3