topic: bring back oldbranchmap context manager for hg 4.9 This is only needed in Mercurial 4.9 because branchcache class there tries to use super() in its __init__() and it results in an infinite recursion because we also implement __init__() with super() in _topiccache and then replace the original branchmap.branchcache class with our own. At least there are now compatibility comments to help remove this hack ASAP.