Slow git clone

Some of the git clones are pretty slow (in particular Linux).

One opportunity to speed up would be to pass additional flags to git clone from the repo entry to enable use of blob-less, tree-less or shallow clones (see https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/).

Another opportunity that is a little bit more specific is if you have the same git repositories in multiple layers or different stacks (or like to restart from scratch regularly), it would be nice to have an command line option that would enable caching of git clones. One option is to always clone in the cache directory first, and then sync to the source directory. Another option would be to leverage something like git-new-workdir. git-new-workdir is the predecessor to git-worktree (which I am pretty sure would not work for this).