Perforce: Can't diff files in Perforce Submit window when using P4CONFIG

Qt Creator 3.5.0 master branch has P4CONFIG support, which means one can work on multiple projects in different workspaces without having to configure Qt Creator every time.

Unfortunately when diffing files in the Submit Window the logic is a bit different and P4CONFIG is not taken into consideration resulting in a nice:

Perforce client error:
	Connect to server failed; check $P4PORT.
	TCP connect to perforce:1666 failed.
	No such host is known.