Fix CreateCommit

The input and response types for commit messages differ in the API.
While commit.Tree and commit.Parents are objects with fields when
returned by the GitHub API, the attributes are plain strings when
creating new commits.

See: https://developer.github.com/v3/git/commits/#create-a-commit

github/git_commits.go

@@ -57,6 +57,15 @@ func (s *GitService) GetCommit(owner string, repo string, sha string) (*Commit,
 	return c, resp, err
 }
 
+// createCommit represents the body of a CreateCommit request.
+type createCommit struct {
+	Author    *CommitAuthor `json:"author,omitempty"`
+	Committer *CommitAuthor `json:"committer,omitempty"`
+	Message   *string       `json:"message,omitempty"`
+	Tree      *string       `json:"tree,omitempty"`
+	Parents   []string      `json:"parents,omitempty"`
+}
+
 // CreateCommit creates a new commit in a repository.
 //
 // The commit.Committer is optional and will be filled with the commit.Author
@@ -66,7 +75,24 @@ func (s *GitService) GetCommit(owner string, repo string, sha string) (*Commit,
 // GitHub API docs: http://developer.github.com/v3/git/commits/#create-a-commit
 func (s *GitService) CreateCommit(owner string, repo string, commit *Commit) (*Commit, *Response, error) {
 	u := fmt.Sprintf("repos/%v/%v/git/commits", owner, repo)
-	req, err := s.client.NewRequest("POST", u, commit)
+
+	body := &createCommit{}
+	if commit != nil {
+		parents := make([]string, len(commit.Parents))
+		for i, parent := range commit.Parents {
+			parents[i] = *parent.SHA
+		}
+
+		body = &createCommit{
+			Author:    commit.Author,
+			Committer: commit.Committer,
+			Message:   commit.Message,
+			Tree:      commit.Tree.SHA,
+			Parents:   parents,
+		}
+	}
+
+	req, err := s.client.NewRequest("POST", u, body)
 	if err != nil {
 		return nil, nil, err
 	}

github/git_commits_test.go

@@ -42,15 +42,25 @@ func TestGitService_CreateCommit(t *testing.T) {
 	setup()
 	defer teardown()
 
-	input := &Commit{Message: String("m"), Tree: &Tree{SHA: String("t")}}
+	input := &Commit{
+		Message: String("m"),
+		Tree:    &Tree{SHA: String("t")},
+		Parents: []Commit{{SHA: String("p")}},
+	}
 
 	mux.HandleFunc("/repos/o/r/git/commits", func(w http.ResponseWriter, r *http.Request) {
-		v := new(Commit)
+		v := new(createCommit)
 		json.NewDecoder(r.Body).Decode(v)
 
 		testMethod(t, r, "POST")
-		if !reflect.DeepEqual(v, input) {
-			t.Errorf("Request body = %+v, want %+v", v, input)
+
+		want := &createCommit{
+			Message: input.Message,
+			Tree:    String("t"),
+			Parents: []string{"p"},
+		}
+		if !reflect.DeepEqual(v, want) {
+			t.Errorf("Request body = %+v, want %+v", v, want)
 		}
 		fmt.Fprint(w, `{"sha":"s"}`)
 	})