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>

contrib/completion/git-completion.bash

@@ -1833,7 +1833,7 @@ _git_mergetool ()
 		return
 		;;
 	--*)
-		__gitcomp "--tool= --prompt --no-prompt"
+		__gitcomp "--tool= --prompt --no-prompt --gui --no-gui"
 		return
 		;;
 	esac