vcs: dedup auth code between Hg and Git middleware There's a lot of verbatim duplicated code in the Hg and Git VCS middleware. This attempts to deduplicate a bit of it. The _authorize function is a bit awkward, but for now the goal is simply to remove duplicated code, not improving program structure and design. As such, the code in _authorize is almost a verbatim copy of the code removed in the two controllers.