Rework checkout mechanism again, again about grouse

[capnfabs]

Ok, picking this up again. Rather than rely upon the hardlink behaviour, I'm going to do git clone --shared:

When the repository to clone is on the local machine, instead of using hard links, automatically setup .git/objects/info/alternates to share the objects with the source repository. The resulting repository starts out without any object of its own.

This works regardless of whether the temporary directory is on the same mount or not, which is a major advantage of relying upon hardlinks ✨

[capnfabs]

Wow, this is working really well and going much faster than I'd expected. I think having five months of noodling around in Rust code has taught me some pretty good habits, and a lot of the skills were transferable.

[capnfabs]

Still todo:

• Add the hack to swap back in the original remote on submodules after clone
• Fix the tests, make the test suite more comprehensive and also not reliant upon 180MB of remote repos 🙃

[capnfabs]

Just some logging improvements left!

[capnfabs]

Merged.