immutable commits: remove the hint if trying to edit the root commit

The hint is a bit misleading in this case. I also changed the message slightly.

cli/src/cli_util.rs

@@ -1268,10 +1268,19 @@ Set which revision the branch points to with `jj branch set {branch_name} -r <RE
         let revset = self.evaluate_revset(to_rewrite_revset.intersection(&immutable_revset))?;
         if let Some(commit) = revset.iter().commits(self.repo().store()).next() {
             let commit = commit?;
-            return Err(user_error_with_hint(
-                format!("Commit {} is immutable", short_commit_hash(commit.id()),),
-                "Configure the set of immutable commits via `revset-aliases.immutable_heads()`.",
-            ));
+            let error = if commit.id() == self.repo().store().root_commit_id() {
+                user_error(format!(
+                    "The root commit {} is immutable",
+                    short_commit_hash(commit.id()),
+                ))
+            } else {
+                user_error_with_hint(
+                    format!("Commit {} is immutable", short_commit_hash(commit.id()),),
+                    "Configure the set of immutable commits via \
+                     `revset-aliases.immutable_heads()`.",
+                )
+            };
+            return Err(error);
         }
 
         Ok(())

cli/tests/test_immutable_commits.rs

@@ -56,8 +56,7 @@ fn test_rewrite_immutable_generic() {
     test_env.add_config(r#"revset-aliases."immutable_heads()" = "none()""#);
     let stderr = test_env.jj_cmd_failure(&repo_path, &["edit", "root()"]);
     insta::assert_snapshot!(stderr, @r###"
-    Error: Commit 000000000000 is immutable
-    Hint: Configure the set of immutable commits via `revset-aliases.immutable_heads()`.
+    Error: The root commit 000000000000 is immutable
     "###);
     // Error if we redefine immutable_heads() with an argument
     // TODO: This error comes from the built-in definition of

cli/tests/test_new_command.rs

@@ -400,8 +400,7 @@ fn test_new_insert_before_root() {
     let stderr =
         test_env.jj_cmd_failure(&repo_path, &["new", "--insert-before", "-m", "G", "root()"]);
     insta::assert_snapshot!(stderr, @r###"
-    Error: Commit 000000000000 is immutable
-    Hint: Configure the set of immutable commits via `revset-aliases.immutable_heads()`.
+    Error: The root commit 000000000000 is immutable
     "###);
 }