Charter v68: a release branch can be cut from a tag without an exception

• A release branch can be cut from a tag without an exception (#482, reported by
  another project's CTO). Since v66 the pre-push gate refused every push to a declared release branch,
  even one that only created it at a released commit, so each patch release needed an exception
  from the CEO. §2.3: a push that creates a release branch at a commit that the remote's default
  branch, one of its release branches or one of its tags already holds now goes through with no
  exception and runs no check, which covers a tag whose release branch was merged and deleted. One
  that moves a release branch, or creates it at a commit that only a feature branch holds, is
  refused as before (found by QA in its review of PR #484). The README says how to cut one from
  its tag.
• A push of a commit that a remote tag holds runs no check (#482). The gate now lists the
  remote's tags as well as its branches, and asks git once whether any of them holds the pushed
  commit, so the tags add no measurable time (QA's finding in its review of PR #484).
• Upgrading from v67: nothing to do by hand. Restart the three sessions, since §2.3 changes.