deb-git-version-gen: Add --date argument for specifying the changelog entry date
This commit introduces the --date argument to the deb-git-version-gen command, allowing users to specify a specific date which is passed to debchange command for the changelog entry being edited.
Example usage:
$ deb-git-version-gen --distro="apertis" --date="Fri, 30 Jun 2023 13:55:28 +0530" --dch