cli: Add 'dup' alias to 'duplicate' command

CHANGELOG.md

@@ -56,6 +56,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * A new global flag `--ignore-immutable` lets you rewrite immutable commits.
 
+* `jj duplicate` now has a shortened alias `jj dup`. 
+
 ### Fixed bugs
 
 * Revsets now support `\`-escapes in string literal.

cli/src/commands/duplicate.rs

@@ -26,6 +26,7 @@ use crate::ui::Ui;
 
 /// Create a new change with the same content as an existing one
 #[derive(clap::Args, Clone, Debug)]
+#[command(visible_aliases = &["dup"])]
 pub(crate) struct DuplicateArgs {
     /// The revision(s) to duplicate
     #[arg(default_value = "@")]