Bug 938222 - Show hash or other VCS identifier of the indexed source

See the discussion on bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=938222

## TODO

- [x] per-path hash and author lookup on Git
- [ ] per-path hash and author lookup on Hg
- [ ] per-path hash and author lookup on P4
- [x] VCS Hash on each compiled page (+ author)
- [x] VCS Hash on dir listings (+ author)


I haven't yet decided what I think of the general-metadata approach, but this part is definitely weird. I would expect `metadata` to be a dict, avoiding the laborious lookup here.