Ignore *.pid.lock

Some popular applications (for example, Phusion Passenger) leave *.pid.lock file(s). For example, when you type passenger start with simple express app listening on port 3000, it leaves the following files:

• passenger.3000.pid (removed when passenger stops)
• passenger.3000.log
• passenger.3000.pid.lock

While *pid and *.log are ignored, *.pid.lock remains unignored. Phusion Passenger is quite popular, and *.pid.lock file should be ignored.