include --edition in the compiletest stamp

Author: pietroalbini

Diff for: src/tools/compiletest/src/runtest.rs

@@ -177,6 +177,7 @@ pub fn compute_stamp_hash(config: &Config) -> String {
     let mut hash = DefaultHasher::new();
     config.stage_id.hash(&mut hash);
     config.run.hash(&mut hash);
+    config.edition.hash(&mut hash);
 
     match config.debugger {
         Some(Debugger::Cdb) => {