Auto merge of #3048 - RalfJung:fmt-skip-children, r=RalfJung

add '--skip-children' to rustfmt invocation

This finally fixes the issue that we format the same file many times (and `./miri fmt --check` shows duplicate diffs). :)

src/tools/miri/miri-script/src/commands.rs

@@ -510,7 +510,7 @@ fn fmt(flags: Vec<OsString>) -> Result<()> {
 
         let mut cmd = cmd!(
             e.sh,
-            "rustfmt +{toolchain} --edition=2021 --config-path {config_path} {flags...}"
+            "rustfmt +{toolchain} --edition=2021 --config-path {config_path} --unstable-features --skip-children {flags...}"
         );
         eprintln!("$ {cmd} ...");