add deprecated "rename-branch" placeholder command that calls "rename"

[kevgo]

To make it easier for Git Town users to adjust to https://github.com/git-town/git-town/issues/4038, re-introduce the "rename-branch" command temporarily to run the "rename" functionality and print a deprecation message.

• use the undocumented deprecated new-pull-request command as a template
• This command is hidden, so it doesn’t show up anywhere.
• No need to write end-to-end tests for it since it’ll be removed in one of the next breaking changes.