[GHC] #15293: Set up staging branch

[GHC]

#15293: Set up staging branch
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           Reporter:  bgamari        |             Owner:  bgamari
               Type:  task           |            Status:  new
           Priority:  highest        |         Milestone:  8.6.1
          Component:  Continuous     |           Version:  8.4.3
  Integration                        |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Goal: Keep commits that would break CI out of `master`.

 Approach: Introduce a staging branch where commits will first land to go
 through the full CI workup. When a commit passes on `staging`, merge it
 into `master`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15293>
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