fix a case where file tracked by git unexpectedly becomes annex pointer file

smudge: When annex.largefiles=anything, files that were already stored in
git, and have not been modified could sometimes be converted to being
stored in the annex. Changes in 7.20191024 made this more of a problem.
This case is now detected and prevented.

Git/UnionMerge.hs

@@ -31,7 +31,7 @@ import Git.FilePath
  -}
 merge :: Ref -> Ref -> Repo -> IO ()
 merge x y repo = do
-	hashhandle <- hashObjectStart repo
+	hashhandle <- hashObjectStart True repo
 	ch <- catFileStart repo
 	streamUpdateIndex repo
 		[ lsTree x repo