Add support for Git proxy through system config.

This adds support for overriding the HTTP proxy for Git by setting the
system configuration. It uses the git command itself to write to the
config file.
Add support for Subversion HTTP proxies.

I ran into this while attempting to build MRI 1.8.7 which seems to
checkout from the old Subversion repository. At any rate unfortunately
ti seems that Subversion itself doesn't have a `config` command so we're
left with simply writing it directly to /etc/subversion/servers.