Fixes Ben's bug 00f26f04-9202-4288-8744-b29abc2342d6.

I also tweaked update_copyright.sh to make possible future
copyright-blurb revision easier.  The new algorithm is greedier,
overwriting _all_ consecutive comments after a '^# Copyright' line, so
do
  # Copyright
  #   GPL ... GPL ... GPL
  
  # Your comment here...
not
  # Copyright
  #   GPL ... GPL ... GPL
  #
  # Your comment here...
Without the blank line, your comment would get overwritten by the next
run of update_copyright.sh.

Note that catmutt is ignored by update_copyright.sh because Moritz
Barsnick has only licensed his grepm code under the GPLv2 (not
GPLv>=2).  See the initial catmutt commit for details.