Refactoring CreateTestRepository

Now each git command is a function
Perhaps this -- and other concatenations below -- could be simplified:
```go
func CreateEmptyFile(tmpPath, repo, file string) error {
	testPath := path.Join(tmpPath, repo+".git")
	if file == "" {
		file = fmt.Sprintf("README_%d", time.Now().UnixNano())
	}
	content := ""
	return ioutil.WriteFile(path.Join(testPath, file), []byte(content), 0644)
}
```