mergetool: accept -g/--[no-]gui as arguments

mergetool: accept -g/--[no-]gui as arguments

In line with how difftool accepts a -g/--[no-]gui option, make mergetool
accept the same option in order to use the `merge.guitool` variable to
find the default mergetool instead of `merge.tool`.

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>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>