Add a reflog to undo changes

Dev: https://dev.gitlab.org/gitlab/gitlabhq/issues/1458

Inspired by http://gitblit.com/administration.html#H12

It allows you to undo a force push according the the author of GitBlit http://gitblit.com/releases.html#1.3.1 Use trash icon in Gitblit Reflog for branch and tag deletion Update Gitblit Reflog on branch deletion from web UI

Proposed location: refs/meta/reflog

cc/ @sytses @dzaporozhets