chore: add an exmaple: clone with ssh

Author: fatelei

_examples/README.md

@@ -10,6 +10,7 @@ Here you can find a list of annotated _go-git_ examples:
       using a username and password.
     - [personal access token](clone/auth/basic/access_token/main.go) - Cloning
       a repository using a GitHub personal access token.
+    - [ssh private key](clone/auth/ssh/main.go) - Cloning a repository using a ssh private key.
 - [commit](commit/main.go) - Commit changes to the current branch to an existent repository.
 - [push](push/main.go) - Push repository to default remote (origin).
 - [pull](pull/main.go) - Pull changes from a remote repository.

_examples/clone/auth/ssh/main.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	git "github.com/go-git/go-git/v5"
+	. "github.com/go-git/go-git/v5/_examples"
+	"github.com/go-git/go-git/v5/plumbing/transport/ssh"
+)
+
+func main() {
+	CheckArgs("<url>", "<directory>", "<private_key_file>")
+	url, directory, privateKeyFile := os.Args[1], os.Args[2], os.Args[3]
+	var password string
+	if len(os.Args) == 5 {
+		password = os.Args[4]
+	}
+
+	_, err := os.Stat(privateKeyFile)
+	if err != nil {
+		Warning("read file %s failed %s\n", privateKeyFile, err.Error())
+		return
+	}
+
+	// Clone the given repository to the given directory
+	Info("git clone %s ", url)
+	publicKeys, err := ssh.NewPublicKeysFromFile("git", privateKeyFile, password)
+	if err != nil {
+		Warning("generate publickeys failed: %s\n", err.Error())
+		return
+	}
+
+	r, err := git.PlainClone(directory, false, &git.CloneOptions{
+		// The intended use of a GitHub personal access token is in replace of your password
+		// because access tokens can easily be revoked.
+		// https://help.github.com/articles/creating-a-personal-access-token-for-the-command-line/
+		Auth:     publicKeys,
+		URL:      url,
+		Progress: os.Stdout,
+	})
+	CheckIfError(err)
+
+	// ... retrieving the branch being pointed by HEAD
+	ref, err := r.Head()
+	CheckIfError(err)
+	// ... retrieving the commit object
+	commit, err := r.CommitObject(ref.Hash())
+	CheckIfError(err)
+
+	fmt.Println(commit)
+}