subrepo: convert EOL of hgrc before writing to bytes IO
Follows up f2f0a777b2e2. Text IO is useless on Python 3 as it must be a
unicode stream.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true