feat: implement --new flag

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/lib_executor.rs

@@ -263,6 +263,33 @@ impl JjExecutor for JjLib {
             })?;
         Ok(commit.description().trim_end().to_string())
     }
+
+    async fn new_revision(&self, revset: &str) -> Result<(), Error> {
+        let commit_id = self.get_commit_id(revset).await?;
+        let repo = self.repo.lock()?.clone();
+        let mut tx = repo.start_transaction();
+        let parent_commit =
+            tx.repo()
+                .store()
+                .get_commit(&commit_id)
+                .map_err(|e| Error::JjOperation {
+                    context: e.to_string(),
+                })?;
+        tx.repo_mut()
+            .check_out(self.workspace_name.clone(), &parent_commit)
+            .await
+            .map_err(|e| Error::JjOperation {
+                context: e.to_string(),
+            })?;
+        let new_repo =
+            tx.commit("jj-cz: create new revision")
+                .await
+                .map_err(|e| Error::JjOperation {
+                    context: e.to_string(),
+                })?;
+        *self.repo.lock()? = new_repo;
+        Ok(())
+    }
 }
 
 #[cfg(test)]