blackbox: adds a blackbox extension Adds a blackbox extension that listens to ui.log() and writes the messages to .hg/blackbox.log. Future commits will use ui.log() to log commands, unhandled exceptions, incoming changes, and hooks. The extension defaults to logging everything, but can be configured via blackbox.track to only log certain events. Log lines are of the format: "date time user> message" Example log line: 2013/02/09 08:35:19 durham> 1 incoming changes - new heads: d84ced58aaa