refactor(repo): create repo package

- Refactor `installer.clone` into new `repo.Clone` package and method.

- Refactor `sheets.isGitDir` into `repo.GitDir`.

Both of these changes read better, and will facilitate cleaner
architecture when `--update` is implemented.

internal/repo/clone.go

@@ -0,0 +1,25 @@
+package repo
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/go-git/go-git/v5"
+)
+
+// Clone clones the repo available at `url`
+func Clone(url string) error {
+
+	// clone the community cheatsheets
+	_, err := git.PlainClone(url, false, &git.CloneOptions{
+		URL:      "https://github.com/cheat/cheatsheets.git",
+		Depth:    1,
+		Progress: os.Stdout,
+	})
+
+	if err != nil {
+		return fmt.Errorf("failed to clone cheatsheets: %v", err)
+	}
+
+	return nil
+}