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 <redacted>
Signed-off-by: Anmol Mago <redacted>
Signed-off-by: Brian Ho <redacted>
Signed-off-by: David Lu <redacted>
Signed-off-by: Ryan Wang <redacted>
Acked-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>