Clear out markers if they're already in the output.

This can occur if we catch an exception (which had markers)
and then embed the error string into another error string.
If the test runner picks up the nested marker that's disaster.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Cabal/Distribution/Simple/Utils.hs

@@ -626,8 +626,17 @@ withMetadata marker tracer verbosity x = withFrozenCallStack $
                    | otherwise
                    -> id
         NeverMark  -> id)
+    -- Clear out any existing markers
+    . clearMarkers
     $ x
 
+clearMarkers :: String -> String
+clearMarkers s = unlines . filter isMarker $ lines s
+  where
+    isMarker "-----BEGIN CABAL OUTPUT-----" = False
+    isMarker "-----END CABAL OUTPUT-----"   = False
+    isMarker _ = True
+
 -- -----------------------------------------------------------------------------
 -- rawSystem variants
 maybeExit :: IO ExitCode -> IO ()