[GHC] #13590: Update Cabal submodule to whatever the final Cabal 2.0 release is

[GHC]

#13590: Update Cabal submodule to whatever the final Cabal 2.0 release is
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           Reporter:  ezyang         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  libraries      |           Version:  8.2.1-rc2
  (other)                            |
           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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 This is a release blocker and I want to file it here so we don't forget.

 In fact, 2.0 head has already diverged from 8.2.1-rc1, see
 https://github.com/haskell/cabal/issues/4453 for some fall out (since we
 now have multiple "2.0" Cabals floating around.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13590>
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