This PR first implements a `git check-attr` pipe reader - using `git check-attr --stdin -z --cached` - taking account of the change in the output format in git 1.8.5 and creates a helper function to read a tree into a temporary index file for that pipe reader.
It then wires this in to the language stats helper and into the git diff generation.
Files which are marked generated will be folded by default.
Fixes#14786
zeripath