sudo fix

Hi,

This modification was needed to me to fix that: https://github.com/jbox-web/redmine_git_hosting/issues/92

The most important part is this:

```
< test = %x[#{shell_cmd_runner} sudo -nu
> test = %x[#{shell_cmd_runner} sudo -inu
```

The plugin does "sudo -i" everywhere but not here. It's a good idea to do that everywhere. With this the test is representative (and works as expected).

The part

```
< echo "yes"
> whoami
```

is less critical, but testing one word is better than failing to "escapement hell".

Outside this test, commands are run though a script that escapes correctly.