feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

internal/installer/clone.go

@@ -3,19 +3,20 @@ package installer
 import (
 	"fmt"
 	"os"
-	"os/exec"
-)
 
-const cloneURL = "https://github.com/cheat/cheatsheets.git"
+	"github.com/go-git/go-git/v5"
+)
 
 // clone clones the community cheatsheets
 func clone(path string) error {
 
-	// perform the clone in a shell
-	cmd := exec.Command("git", "clone", cloneURL, path)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	err := cmd.Run()
+	// clone the community cheatsheets
+	_, err := git.PlainClone(path, 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)
 	}