Performance improvement

Hi there. I'm the author of [ag](https://github.com/ggreer/the_silver_searcher). I downloaded and compiled grab after seeing it on HN today. I quickly found a bit of low-hanging fruit: FileGrep::find is calling pthread_mutex_lock/unlock for every file searched, even if there are no results to print. I added some simple logic to avoid unnecessary mutex locking. In my benchmarks, this change speeds up grab by about 25%.