Add branch information to short status

When running 'git number -s', the branch is now shown. This is the same as running:

```
$ git status --short --branch
```

The only change that has been made here is that the '--branch' option is now passed to git status.

Note that this is simpler than my first pull request a few weeks ago. Less duplicated code and I'm hoping the tests don't need to change. If you can tell me how to run the perl unit tests that you have, I am happy to verify they still work afterwards.