pkgctl repo clone: add option to switch working tree

As requested in &7 (comment 87782), this implements a --version flag for pkgctl repo clone.

Usecases are:

• CIs like archlinux-repro
• endusers that need an old version of the package (rather secondary)

Example usage: pkgctl repo clone --version="5.2.0-1" bash

This MR is still missing documentation and shell completion, will wait for review before I do that!