Implement Sparse Checkout for GitRepository

[dipti-pai]

Sparse Checkout Directories in GitRepositories.

• Add .spec.sparseCheckout and .status.observedSparseCheckout fields to GitRepository.
• Add controller support to send the sparse checkout directories to go-git via pkg methods.
• Add tests for testing the observed sparse checkout behavior.
• Add docs describing the new fields.

Fixes: #1707

Current behavior worth calling out.

1. If .spec.sparseCheckout directory list includes a directory that does not exist, the underlying go-git checkout method does not return an error and silently continues. See SparseCheckoutDirectories behavior when directory path does not exist go-git/go-git#1500
2. If .spec.sparseCheckout directory list includes directory with more than one level of nesting, sparse checkout is not honored and all directories at top-level are checked out. Will be fixed by SparseCheckoutDirectories works only for 1st level directory. go-git/go-git#1455
3. If .spec.sparseCheckout uses relative path beginning with a ./, the path is ignored and nothing is checked out and no errors are thrown. See SparseCheckoutDirectories ignores relative paths beginning with (./) while checking out git repository go-git/go-git#1506 . This is being handled in source-controller.
   4.. If .spec.sparseCheckout includes an empty directory, the entire repository is checked out (Expected)

[stefanprodan]

If .spec.sparseCheckout uses relative path beginning with a ./, the path is ignored and nothing is checked out and no errors are thrown.

@dipti-pai I think we should handle this on our own. We could trim ./ by prefix, before passing the array to the Git client.

[pjbgf]

From the abnormal behaviours called out, #2 was the only one which there would not be a way to work around straight from the controller. That is now fixed and merged into main.

I'd agree with @stefanprodan on #3, and the fix LGTM.

For #1, we could workaround by adding a os.Lstat after the clone operation to confirm the dir exists - only when sparse checkout is used. @stefanprodan WDYT?

[stefanprodan]

we could workaround by adding a os.Lstat after the clone operation to confirm the dir exists - only when sparse checkout is used

Yes we could verify the dirs and error out in the controller 👍