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)


Asked myself the same thing: if we create a metadata dict we should embrace our decision by indexing the dict directly into ES. I don't know for now, depends on what we do with the "metadata" stuff I think... Maybe metadata is a bad name too...