context: open files in bytes mode

I'm stunned this open() call has survived this long without the b in
the mode - it seems like it should have been a source of bugs somewhere...

Differential Revision: https://phab.mercurial-scm.org/D5080

mercurial/context.py

@@ -2433,5 +2433,5 b' class arbitraryfilectx(object):'
 
     def write(self, data, flags, **kwargs):
         assert not flags
-        with open(self._path, "w") as f:
+        with open(self._path, "wb") as f:
             f.write(data)