Improvement: `git-maintenance` to allow configuring of remotes to fetch

`git-maintenance` aims to improve the health & performance of especially large git repositories. A part of git maintenance tasks is `prefetch`.

However, at the moment, it prefetches all objects from all branches —

This seems non-optimal for repositories where users use the main repository to fork branches (instead of personal forks) as the remote could have thousands of refs and objects from branches that a user may not care about, leading to a very large prefetch dir.

For large repositories, it might instead be helpful to only fetch objects on `main` or `master` because those are the ones most likely to be used by users.

To this effect, git maintenance should perhaps expose a maintenance.prefetch.refs setting that subsets prefetching only to certain references.

Is this something that has been raised before / is in works by any chance or something that would be an acceptable contribution?