git-buildpackage should run 'setup_pbuilder' before prebuild

[neutrico]

Currently git-buildpackage runs prebuild command (if any) and then setup_pbuilder which sets DIST and ARCH env variables.

So, we cannot use these variables in prebuild script, while this could be very handy. Because with these variables it could be available to create or update pbuilder environment before running git-pbuilder.

It is possible to set these variables like this:

DIST=oneiric git-buildpackage --git-pbuilder --git-arch=oneiric

but this is misleading because this way we can also do something like:

DIST=precise git-buildpackage --git-pbuilder --git-arch=oneiric