bash completion: Adjust to new 'guix help' output.

This is a followup to 3794ce93be8216d8378df7b808ce7f53b1e05a53, which
broke command completion.

Reported by Oleg Pykhalov <go.wigust@gmail.com>.

* etc/completion/bash/guix (_guix_complete_command): Update to match
output of 'guix help' as introduced in
3794ce93be8216d8378df7b808ce7f53b1e05a53.

etc/completion/bash/guix

@@ -27,8 +27,9 @@ _guix_complete_command ()
     if [ -z "$_guix_commands" ]
     then
 	# Cache the list of commands to speed things up.
-	_guix_commands="$(guix --help 2> /dev/null \
-                                     | grep '^  ' | cut -c 2-)"
+	_guix_commands="$(guix --help 2> /dev/null  \
+                                     | grep '^    ' \
+				     | sed '-es/^ *\([a-z-]\+\).*$/\1/g')"
     fi
     COMPREPLY=($(compgen -W "$_guix_commands" -- "$word_at_point"))
 }