Incorporate NMU (bash-completion 1:2.8-1.1)

Emmanouil Kampitakis prepared an NMU (versioned as 1:2.8-1.1) [1] and
uploaded it to Debian [2].  This patch incorporates the changes provided
by the NMU.

Thanks to Emmanouil Kampitakis.

[1] https://bugs.debian.org/910128#26
[2] https://bugs.debian.org/910128#36