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tests: change the fixer commands to use the buffer attribute on stdio objects

Authored by mharbison72.

Description

tests: change the fixer commands to use the buffer attribute on stdio objects

Otherwise \r was getting injected into the fixed lines and throwing off the
commit hashes on Windows when the fixer is invoked with py3.

Differential Revision: https://phab.mercurial-scm.org/D10637