Support git_repository_message, git_repository_message_remove (#734)

Closes #646

merge_test.go

@@ -43,6 +43,14 @@ func TestMergeWithSelf(t *testing.T) {
 	mergeHeads[0] = mergeHead
 	err = repo.Merge(mergeHeads, nil, nil)
 	checkFatal(t, err)
+
+	mergeMessage, err := repo.Message()
+	checkFatal(t, err)
+
+	expectedMessage := "Merge branch 'master'\n"
+	if mergeMessage != expectedMessage {
+		t.Errorf("merge Message = %v, want %v", mergeMessage, expectedMessage)
+	}
 }
 
 func TestMergeAnalysisWithSelf(t *testing.T) {

repository.go

@@ -688,3 +688,39 @@ func (r *Repository) ClearGitIgnoreRules() error {
 	}
 	return nil
 }
+
+// Message retrieves git's prepared message.
+// Operations such as git revert/cherry-pick/merge with the -n option stop just
+// short of creating a commit with the changes and save their prepared message
+// in .git/MERGE_MSG so the next git-commit execution can present it to the
+// user for them to amend if they wish.
+//
+// Use this function to get the contents of this file. Don't forget to remove
+// the file after you create the commit.
+func (r *Repository) Message() (string, error) {
+	buf := C.git_buf{}
+	defer C.git_buf_dispose(&buf)
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	cErr := C.git_repository_message(&buf, r.ptr)
+	runtime.KeepAlive(r)
+	if cErr < 0 {
+		return "", MakeGitError(cErr)
+	}
+	return C.GoString(buf.ptr), nil
+}
+
+// RemoveMessage removes git's prepared message.
+func (r *Repository) RemoveMessage() error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	cErr := C.git_repository_message_remove(r.ptr)
+	runtime.KeepAlive(r)
+	if cErr < 0 {
+		return MakeGitError(cErr)
+	}
+	return  nil
+}