Configure canonical /bin/bash path

Simon McVittierequested to merge
smcv/quilt:merged-usr into master

From: Vivek Das Mohapatra vivek@collabora.com

This means that quilt binary packages built in a merged-/usr environment continue to work on non-merged-/usr systems.

Closes: #913226 Reviewed-by: Simon McVittie smcv@collabora.com

Vivek's patch from https://bugs.debian.org/913226, plus a changelog entry. I've tested this in a merged-/usr schroot and confirmed it works.

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