Worktree Add function adds ".git" directory

[tkrisko]

Hi There

The below code cause ".git" is added to the index.

func TestAdd(t *testing.T) {
   repoPath, err := ioutil.TempDir("", "TestAdd")
   if err != nil {
   	t.Fatal(err)
   }
   repo, err := git.PlainInit(repoPath, false)
   if err != nil {
   	t.Fatal(err)
   }

   w, err := repo.Worktree()
   if err != nil {
   	t.Fatal(err)
   }
   _, err = w.Add(".")
   if err != nil {
   	t.Fatal(err)
   }
   w.Commit("Test Add", &git.CommitOptions{Author: getSignature()})
   iter, err := repo.Log(&git.LogOptions{})
   err = iter.ForEach(func(c *object.Commit) error {
   	files, err := c.Files()
   	if err != nil {
   		t.Fatal(err)
   	}
   	err = files.ForEach(func(f *object.File) error {
   		t.Log(f.Name())
   		return nil
   	})
   	return nil
   })
}

func getSignature() *object.Signature {
   when := time.Now()
   sig := &object.Signature{
   	Name:  "foo",
   	Email: "foo@foo.foo",
   	When:  when,
   }
   return sig
}

I would assume the Add will ignore ".git" directory. However the output looks like:

=== RUN   TestAdd
--- PASS: TestAdd (0.01s)
	bogus_test.go:38: .git/HEAD
	bogus_test.go:38: .git/objects/cb/089cd89a7d7686d284d8761201649346b5aa1c
PASS
ok  	github.kohls.com/EnterpriseDevOps/go-git2consul/repository	0.027s

[kuba--]

Hopefully it can be fixed by: #815