bash completion: use builtin filenames option (#4648)

Use a builtin bash option which detects filenames in completion outputs
and reflects this in sensible <tab> completion behaviour.

contrib/completion/hx.bash

@@ -19,5 +19,5 @@ _hx() {
 		COMPREPLY=($(compgen -fd -W "-h --help --tutor -V --version -v -vv -vvv --health -g --grammar --vsplit --hsplit -c --config --log" -- $2))
 		;;
 	esac
-} && complete -F _hx hx
+} && complete -o filenames -F _hx hx