Previously "bare\r" was split into ["bare"] even though the
documentation said that only LF and CRLF count as newlines.
This fix is a behavioural change, even though it brings the behaviour
into line with the documentation, and into line with that of
`std::io::BufRead::lines()`.
This is an alternative to #91051, which proposes to document rather
than fix the behaviour.
Fixes#94435.
Co-authored-by: Ian Jackson <ijackson@chiark.greenend.org.uk>