Feature request: add cacheing

[dabrahams]

I have to check out the same large repository (LLVM) in two different jobs and many matrix configurations in my workflow, and it's nearly always checked out at the same commit. If I could use a cache for that repo, it would speed up CI considerably.
git ls-remote https://github.com/llvm/llvm-project.git -t llvmorg-$LLVM_VERSION | cut -f 1 could be used as a cache key. I may be able to figure out how to get this to work on my own, but it would be awesome if the functionality were built in.

Thanks!