Only attempt to `rustfmt` files checked into git

Only attempt to `rustfmt` files checked into git

This avoids `rustfmt` failing on Rust files generated by dependent
crates in `target`, eg

```
+ rustfmt --edition 2021 --check ./target/debug/build/thiserror-8230374e07b5c05a/out/probe.rs
Diff in /home/matt/rust-lightning-3/target/debug/build/thiserror-8230374e07b5c05a/out/probe.rs at line 1:

-    #![feature(provide_any)]
+#![feature(provide_any)]

-    use std::any::{Demand, Provider};
+use std::any::{Demand, Provider};

-    fn _f<'a, P: Provider>(p: &'a P, demand: &mut Demand<'a>) {
-        p.provide(demand);
-    }
+fn _f<'a, P: Provider>(p: &'a P, demand: &mut Demand<'a>) {
+	p.provide(demand);
+}
```