Function git_diff_file 

pub fn git_diff_file(
    repo: &Path,
    from_ref: &str,
    to_ref: &str,
    file_path: &str,
) -> Option<String>

Get the diff of a single file between two refs via git diff <from>..<to> -- <path>.

Returns None if the file has no changes between the refs, the git command fails, or the output is empty. All failures are logged at trace level.