Fix unary operator expected

Added in a way to easily inspect just the output of the command under the scope to more easily catch things like this

```
cucumber features/git-prune-branches.feature OUTPUT_COMMANDS=1
```

Tacked on a couple more commits fixing bugs founds using OUTPUT_COMMANDS
* Git extract had some similar errors
* Abort / continue checks weren't working properly (git extract was listing a continue message and trying to remove a non existent continue file)
The parameters of these two actually don't match exactly. The big difference is debug defaults to ENV['OUTPUT_COMMANDS'] in this function.