fix bash completion hangs (Closes: #776240)

Apply the patch sent in https://bugs.debian.org/776240#23

For the avoidance of doubt, I do not think the changes in the patch are long and creative enough to be copyrighted. Should this turn out to be wrong, I hereby release my contribution under the same terms as the original file (GPL-2+, according to the debian/copyright file), as I am sure the other contributors who sent patches to bug #776240 have intended to...