completion: support `git mergetool --[no-]gui`

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Anmol Mago <anmolmago@gmail.com>
Signed-off-by: Brian Ho <briankyho@gmail.com>
Signed-off-by: David Lu <david.lu97@outlook.com>
Signed-off-by: Ryan Wang <shirui.wang@hotmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>