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

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Anmol Mago <[email protected]>
Signed-off-by: Brian Ho <[email protected]>
Signed-off-by: David Lu <[email protected]>
Signed-off-by: Ryan Wang <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

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