[GHC] #15451: Fix Git commit ID detection in worktrees

[GHC]

#15451: Fix Git commit ID detection in worktrees
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           Reporter:  hsyl20         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Build System   |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D5016     |         Wiki Page:
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 When using a Git worktree, ".git" is a file, not a directory, hence
 "configure" doesn't detect it correctly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15451>
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