[git-buildpackage] how can I use an alternate pbuilderrc with gbp?

[Ken Dreyer]

Currently "gbp buildpackage" reads my ~/.pbuilderrc. What is the best
way to make it read an alternate (arbitrary) file? Something like
/tmp/my/own/pbuilderrc

I see there is a "--configfile" option to pbuilder, but I am not sure
how to use it with "gbp buildpackage" ? I've tried a couple approaches
without success. I promise to send a patch to the docs if someone can
clue me in :)

- Ken