Support sparse checkouts

Purpose

When using large project with many modules (AKA monorepo) being able to do sparse checkouts (supported since git 2.25.x) would be very helpful.

Example

Repo containing:

scripts
tools
modules/mobile-client
modules/server
modules/web

We want to only checkout and build the mobile-client (nice when combined with paths/paths_ignore). So the equivalent of git sparse-checkout init --cone && git sparse-checkout set modules/mobile-client scripts tools would be:

- name: Checkout tools repo
  uses: actions/checkout@v3
  with:
    repository: my-org/my-repo
    sparse:
      - modules/mobile-client
      - scripts
      - tools

Ref: https://github.blog/2020-01-17-bring-your-monorepo-down-to-size-with-sparse-checkout/

May 21 '22 10:05 devminded

Related #172 #680

May 21 '22 10:05 devminded

Worth noting that this https://github.com/actions/checkout/blob/main/src/git-source-provider.ts#L188 causes all LFS files to be retrieved for the given branch.

When using sparse-checkout AND lfs, checkout will only download lfs files as needed so the sparse rules can be used to prevent a lot of stuff being downloaded which is great, but only with the call to git lfs fetch origin ... being removed.