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rust-chg: have attach_io() simply take reference of AsRawFd object

Authored by yuja.

Description

rust-chg: have attach_io() simply take reference of AsRawFd object

We no longer have to deal with the restriction of the Future type.
Before, these file objects couldn't be references and that's the only
reason why we had to make stderr an Option<T>.

This fixes future type deduction issue of stderr = None, where rustc would
complain that T of Option<T> couldn't be deduced.

Differential Revision: https://phab.mercurial-scm.org/D8443