Closing SourceTree leaves Git child processes running

If you open the SourceTree application, and have enabled the "Check default remotes for updates every X minutes" feature, Git `fetch` processes can be started in the background that invoke credential helpers that can wait for user interaction to complete.

Closing the main SourceTree application leaves these `git fetch` processes alive (and their children).

When the main application is closed, all child Git processes should also be terminated and not left as orphans.