Enforce LF line endings on checkout

[JamieMagee]

Since #6682 all files in the repository have been normalized to LF. However, when checking out the repository on Windows, and core.autocrlf or core.eol are unset, line endings are set to CRLF. See the documentation¹:

If the eol attribute is unspecified for a file, its line endings in the working directory are determined by the core.autocrlf or core.eol configuration variable (see the definitions of those options in git-config[1]). If text is set but neither of those variables is, the default is eol=crlf on Windows and eol=lf on all other platforms.

We should default to a known good configuration, and not depend on people to have the correct git config to.

Footnotes

1. https://git-scm.com/docs/gitattributes#Documentation/gitattributes.txt-Unspecified-1-1 ↩