rhg: sort unsupported extensions in error message

This caused some flakiness in test output, and is also just better for users.

Differential Revision: https://phab.mercurial-scm.org/D12389

rust/rhg/src/main.rs

@@ -674,6 +674,9 b' fn check_extensions(config: &Config) -> '
     if unsupported.is_empty() {
         Ok(())
     } else {
+        let mut unsupported: Vec<_> = unsupported.into_iter().collect();
+        // Sort the extensions to get a stable output
+        unsupported.sort();
         Err(CommandError::UnsupportedFeature {
             message: format_bytes!(
                 b"extensions: {} (consider adding them to 'rhg.ignored-extensions' config)",