git-annex: work around Joey Hess' latest brain fart

Now we're releasing incomplete Hackage tarballs on purpose:

  http://git-annex.branchable.com/bugs/bash_completion_file_is_missing_in_the_6.20160527_tarball_on_hackage/

You know, I'll fix this shit one last time, but on the next occasion
upstream breaks the build for no apparent reason, I'm going to abandon
this project and leave it to someone else to package, because these kind
of wacko engineering decisions cast a bad light on the soundness of
git-annex as a whole, IMHO.