bash completion: Complete long options with no short variant.

* etc/completion/bash/guix (_guix_complete_option): Change grep regexp
  to match options that don't have a short option name.

etc/completion/bash/guix

@@ -35,7 +35,7 @@ _guix_complete_installed_package ()
 _guix_complete_option ()
 {
     local options="$(${COMP_WORDS[0]} ${COMP_WORDS[1]} --help \
-                            | grep '^  -' \
+                            | grep '^  \+-' \
                             | sed -e's/^.*--\([a-zA-Z0-9_-]\+\)\(=\?\).*/--\1\2/g' )"
     compopt -o nospace
     COMPREPLY=($(compgen -W "$options" -- "${COMP_WORDS[$word_count - 1]}"))