completion: complete '--tool-help' in 'git mergetool'

Signed-off-by: Philippe Blain <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

contrib/completion/git-completion.bash

@@ -2331,7 +2331,7 @@ _git_mergetool ()
 		return
 		;;
 	--*)
-		__gitcomp "--tool= --prompt --no-prompt --gui --no-gui"
+		__gitcomp "--tool= --tool-help --prompt --no-prompt --gui --no-gui"
 		return
 		;;
 	esac