enable git proxying

[Greg Lutostanski]

Useful particularly for snap building.

Motivation: https://lists.ubuntu.com/archives/snapcraft/2017-February/003200.html

Possible path forward: https://stackoverflow.com/questions/783811/getting-git-to-work-with-a-proxy-server/21136254#21136254

[Colin Watson (cjwatson)]

This is a reasonably frequent source of user confusion, so we'd do well to sort it out.

[Joseph Wakeling (webdrake)]

Just to note: while this is an inconvenience for most projects, it's an outright blocker if one is trying to build an external snap based on an upstream that uses submodules identified with `git://` URLs. See e.g.:
https://lists.ubuntu.com/archives/snapcraft/2017-March/003578.html
https://lists.ubuntu.com/archives/snapcraft/2017-March/003577.html

[Leo Arias (elopio)]

npm has a super ugly way to work this around:
https://github.com/npm/npm/wiki/Troubleshooting#npm-only-uses-git-and-sshgit-urls-for-github-repos-breaking-proxies

We have no way to replace the global url in build.snapcraft.io before the pull, so this also seems to be a blocker for many nodejs snaps.

[Colin Watson (cjwatson)]

Fixed in launchpad-buildd 143.