Don't generate superfluous output

Hey @pcreux,

I recently started using `git-branch-delete-orphans` in utility scripts, etc. _(in addition to still using it in the traditional, interactive mode it was originally designed for)_

However, in scripted mode, the command is quite verbose, even and especially if there's nothing to do, ie. there are no orphaned branches that can be deleted, generating the following superfluous messages:

```shell
Listing remote branches...
From git@github.com:foo/bar.git

No orphaned local branches, nothing to do.
```

The changes in this pull request ensure the command is quiet in this case, ie. don't generate any output if there are no orphaned branches... see also the "Rule of Silence" from [the Unix philosophy](http://en.wikipedia.org/wiki/Unix_philosophy):

> **Rule of Silence:** Developers should design programs so that they do not print unnecessary output. This rule aims to allows other programs and developers to pick out the information they need from a program's output without having to parse verbosity.

/cc @waynemoore