git: add support for cloning empty repos

[aryan9600]

Add support for cloning empty repos using go-git and libgit2.
go-git returns an error when cloning an empty repository directly, which is handled by using the advice provided here: go-git/go-git#118 (comment). libgit2 doesn't return an error and thus the change is much simpler.
The go-git package gets test coverage for this scenario but the libgit2 package does not because of the following weird behavior:

_, err := git2go.InitRepository(filepath.Join(server.Root(), emptyRepoPath), true)

err = l.remote.Fetch([]string{branch},
	&git2go.FetchOptions{
		DownloadTags:    git2go.DownloadTagsNone,
		RemoteCallbacks: remoteCallBacks,
	},
	"")
if err != nil {
	return nil, fmt.Errorf("unable to fetch remote '%s': %w", url, gitutil.LibGit2Error(err))
}

isEmpty, err := l.repository.IsEmpty()
fmt.Println(isEmpty) // prints false

To make sure the changes to the libgit2 package have the intended effect, it was tested (and confirmed to have the correct behavior) using empty repos on GitHub and GitLab.(same goes for the go-git package as well).
Signed-off-by: Sanskar Jaiswal [email protected]

[pjbgf]

LGTM

thanks @aryan9600! please rebase.

[hiddeco]

Wouldn't it have been better to return a typed error to avoid panics? This would be easy to ignore where applicable using errors.Is. For example:

c, err := r.Clone(context.Background(), "https://github.com/...")
if err != nil && !errors.Is(err, git.ErrEmptyRepository) {
	return err
}

if c == nil {
	// Do something to init repository...
}

// ...Continue

Where as the current solution results in unsafe default behavior, as has become apparent in fluxcd/source-controller#1021