[GHC] #15924: Do not save performance test results if work tree is dirty

[GHC]

#15924: Do not save performance test results if work tree is dirty
-------------------------------------+-------------------------------------
           Reporter:  davide         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Test Suite     |           Version:  8.6.2
           Keywords:  performance    |  Operating System:  Unknown/Multiple
  tests git notes                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  12758
Differential Rev(s):                 |         Wiki Page:
                                     |  Performance/Tests
-------------------------------------+-------------------------------------
 The test driver will automatically save performance results to git notes.
 Notes are attached to commits, but if the working tree is dirty,
 performance characteristics my be affected and the saved results will not
 be valid.

 * Only save performance test results if work tree is dirty.
 * Bonus: don't save to a git note if we have 0 results.
 * Update wiki page.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15924>
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