py3: open chistedit file in binary mode using vfs
We write bytes to the file, so it should be open in binary
mode. Opening it via the vfs takes care of that for us.
Now you'll get yet a different traceback if you try to confirm you
histedit plan.
Differential Revision: https://phab.mercurial-scm.org/D7185
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273ce12ad8f1` will report some style violations on a very old commit).