remotefilelog: rework workaround for sshpeer deadlocks
The wrapping of `sshpeer.cleanup` silently broke when `cleanup` was
renamed to `_cleanup`, a couple of years ago.
I don't know what `orig.im_self` is, but regardless, the intention of
the wrapping seems pretty clear: close stderr before
sshpeer._cleanuppipes blocks on it. So do that.
Differential Revision: https://phab.mercurial-scm.org/D9997
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