tg-completion: fix extraction of available commands from output of help

The completion script extracts the list of available tg
commands from the output of the help command. This wasn't working
(presumably because the output of 'tg help' has changed) and gets
fixed by this commit.

contrib/tg-completion.bash

@@ -203,7 +203,7 @@ __tg_commands ()
 		return
 	fi
 	local i IFS=" "$'\n'
-	for i in $(tg help | sed -n 's/^Usage:.*(\([^)]*\)).*/\1/p' | tr '|' ' ')
+	for i in $(tg help | sed -e '1,/^Available tg commands.*/d' -e 's,\[\|\],,g')
 	do
 		case $i in
 		*--*)             : helper pattern;;