util: give appropriate default args to atomictempfile()

mode='w+b' is the default of python's TemporaryFile().

mercurial/util.py

@@ -772,7 +772,7 b' class atomictempfile(object):'
     file.  When rename is called, the copy is renamed to the original
     name, making the changes visible.
     """
-    def __init__(self, name, mode, createmode):
+    def __init__(self, name, mode='w+b', createmode=None):
         self.__name = name
         self._fp = None
         self.temp = mktempcopy(name, emptyok=('w' in mode),