A Mercurial hook to post to Review Board on push to a central server.

The hook only allows the push if all commits have been approved in a
review requests, otherwise it rejects it and creates a review request
for the (new) commits. This is similar to "rbt post", but

1. does not require the user to install RBTools locally
2. makes sure any changes pushed to the central server have been
   reviewed and approved
3. makes links of all references to tickets/bugs/issues, which
   rbt post doesn't do.
4. automatically finds the right review request to update if there
   are any new commits, based the commit ID and a date/author hash.
   This does not require the user to confirm anything, which rbt post
   (often) requires. This also allows the hook to recognize
   rebased/amended changesets, because the date/author hash
   is unchanged.

Patch is based on work of Halvor Lund!

Replacing this: https://reviews.reviewboard.org/r/7100/
We use this commit hook since february 2015 as a gatekeeper like gerrit codereview.