Why not just use tar or any other archive tool on the repository .git folder? Unless your repository is a un-gc'd mess with millions of unpacked objects...
I think that is a fair alternative, but restoring the backup means that the repository is in a bit of a weird state. Whereas a bundle can be cloned from nicely. Your way does have the property that it includes hooks and config, though (which could be desired or not).
This is also non-deterministic between versions of tar, but I guess for this usecase that would be fine. It’s just not good for reproducible build systems when trying to recreate tarballs after years.
> The naive solution of simply backing up the entire file-system tree is clearly not desirable since that would clutter the backup with useless build artifacts.
Build artifacts can be filtered out with tar --exclude patterns, but this is a language-dependent set that will require curation.
