Adding a Processor for Git info

Adding a Processor for Git Branch and Git Commit information.

**Usage**
```php
use Monolog\Processor\GitProcessor;
...
$log->pushProcessor(new GitProcessor());
```

**Extra Field**
```json
{
    "git":{
        "branch":"development",
        "commit":"c0f9595a478924fe1cc8276d9f7e89af7fa118b8"
    }
}
```