cli: add better error message when immutable_heads() cannot be resolved

cli/src/cli_util.rs

@@ -1012,7 +1012,12 @@ impl WorkspaceCommandHelper {
         let immutable = revset_util::parse_immutable_expression(&self.revset_parse_context())?;
         let mut expression = self.attach_revset_evaluator(immutable)?;
         expression.intersect_with(&to_rewrite_revset);
-        if let Some(commit_id) = expression.evaluate_to_commit_ids()?.next() {
+
+        let mut commit_id_iter = expression.evaluate_to_commit_ids().map_err(|e| {
+            config_error_with_message("Invalid `revset-aliases.immutable_heads()`", e)
+        })?;
+
+        if let Some(commit_id) = commit_id_iter.next() {
             let error = if &commit_id == self.repo().store().root_commit_id() {
                 user_error(format!(
                     "The root commit {} is immutable",

docs/contributing.md

@@ -114,7 +114,7 @@ You will probably also want to make the `gh-pages` branch immutable (and thereby
 hidden from the default `jj log` output) by running the following in your repo:
 
 ```shell
-jj config set --repo "revset-aliases.immutable_heads()" "main@origin | gh-pages@origin"
+jj config set --repo "revset-aliases.immutable_heads()" 'remote_branches(exact:"main") | remote_branches(exact:"gh-pages")'
 ```
 
 ### Summary