Fix error when trying to get diff with nil parent

Every initial commit have no parent, thus if you try to fetch full log, you'll receive error like this:
`git diff '--numstat' 'f5baa11a1c82dc42ade5c291e9f061c13b66bc2f' '' 2>&1:fatal: ambiguous argument ''`

To fix this we can use `git show` command if current object have no parent. This command can accept all options from `git diff`, the only difference is that we should not pass (empty) parent commit hash.