docs(README): update the README to reflect new features

The new `--new` or `-n` flag allows to create a new revision after the
single revision being described.

Running `jj-cz --new` is the equivalent of running `jj-cz --new @`,
describing the current revision and creating a new revision after it.

Passing several revisions to `jj-cz` with the `--new` flag will result
in an error.

src/jj/mock.rs

@@ -300,7 +300,8 @@ mod tests {
     /// Test mock new_revision() returns configured error
     #[tokio::test]
     async fn mock_new_revision_returns_error() {
-        let mock = MockJjExecutor::new().with_new_revision_response(Err(Error::RepositoryLocked));
+        let mock =
+            MockJjExecutor::new().with_new_revision_response(Err(Error::RepositoryLocked));
         let result = mock.new_revision("@").await;
         assert!(result.is_err());
         assert!(matches!(result.unwrap_err(), Error::RepositoryLocked));
@@ -309,7 +310,8 @@ mod tests {
     /// Test mock new_revision() records revset even on error
     #[tokio::test]
     async fn mock_new_revision_records_revset_on_error() {
-        let mock = MockJjExecutor::new().with_new_revision_response(Err(Error::JjOperation {
+        let mock =
+            MockJjExecutor::new().with_new_revision_response(Err(Error::JjOperation {
                 context: "failed".to_string(),
             }));
         let result = mock.new_revision("abc").await;

src/prompts/workflow.rs

@@ -783,8 +783,8 @@ mod tests {
     /// Test workflow new_revision() propagates executor errors
     #[tokio::test]
     async fn workflow_new_revision_propagates_error() {
-        let mock_executor =
-            MockJjExecutor::new().with_new_revision_response(Err(Error::RepositoryLocked));
+        let mock_executor = MockJjExecutor::new()
+            .with_new_revision_response(Err(Error::RepositoryLocked));
         let workflow = CommitWorkflow::new(mock_executor);
 
         let result = workflow.new_revision("@").await;
@@ -809,10 +809,7 @@ mod tests {
         let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
 
         workflow.run_for_revset("@").await.expect("describe failed");
-        workflow
-            .new_revision("@")
-            .await
-            .expect("new_revision failed");
+        workflow.new_revision("@").await.expect("new_revision failed");
 
         let messages = workflow.executor.describe_messages();
         assert_eq!(messages.len(), 1);