Function filter_process 

pub fn filter_process<R, W, F>(
    store: &Store,
    input: R,
    output: W,
    fetch: F,
) -> Result<(), FilterProcessError>
where
    R: Read,
    W: Write,
    F: FnMut(&Pointer) -> Result<(), FetchError>,

Run the filter-process protocol against input/output (typically stdin/stdout). Returns when git closes its end of the pipe.

fetch is called when a smudge request hits an object that isn’t in the local store; see smudge_with_fetch for semantics. If fetching is not supported in the caller’s context, pass a closure that always errors — the protocol will then surface those smudges as status=error to git, same as if smudge hit ObjectMissing.