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 @ && jj new`. Running `jj-cz --new xs` is
the equivalent of running `jj-cz xs && jj new xs`.

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

Refs: #6

src/jj/mod.rs

@@ -21,6 +21,11 @@ pub trait JjExecutor: Send + Sync {
 
     /// Get the current description of a specific revision
     async fn get_description(&self, revset: &str) -> Result<String, Error>;
+
+    /// Create a new empty child revision parented on `revset`.
+    ///
+    /// Equivalent to `jj new <revset>`
+    async fn new_revision(&self, revset: &str) -> Result<(), Error>;
 }
 
 #[cfg(test)]