Set diff=rust git attribute for rust files
Set diff=rust git attribute for rust files

This gives nicer hunk headers

Fixes #1095

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust