Fire GitLab post-receive hook directly from Mercurial
We are currently using a 2-step push to the inner Git repository, soon to be a 3-step (see !20) This is suboptimal for performance, it would be better to notify GitLab of the three kinds of changes (topic branches, non-topic branches, pruning of published topic branches) directly. It would also have these added benefits: - possibility to pass the GitLab user along (currently all pushes seem to have been done by 'root' to GitLab) - stop using SSH internally (less latency, probably) - better transactionality: we could postposne it after the `hg-git` push, and even if needed postpone some of the notifications to a post-transaction hook, by storing extra info in the transaction. All of this requires disabling the regular post-receive hook, simply by removal from the `gitlab-shell`'s hooks subdirectory, and re-fire it from Mercurial (if we have to support Git and Mercurial at this point, then we'd have to use some environment variable to make it a no-op, since it's the same hook for all Git repos).
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