[Merge] lp:~smoser/ubuntu-dev-tools/better-proxy-support into lp:ubuntu-dev-tools

[Stefano Rivera]

I don't see why we need both http_proxy and DEBOOTSTRAP_PROXY?

mk-sbuild is bash, not sh, we don't need to be quite so crazy scared of empty variables. But that just makes it harder to read, I think the code is fine.

Why are we adding Retries = 0?
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https://code.launchpad.net/~smoser/ubuntu-dev-tools/better-proxy-support/+merge/80386
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