revert parentDir change

Reverts 965e106f24

Unfortunately, this caused breakage on Windows, and possibly elsewhere,
because parentDir and takeDirectory do not behave the same when there is a
trailing directory separator.

Git/Repair.hs

@@ -244,7 +244,7 @@ explodePackedRefsFile r = do
   where
 	makeref (sha, ref) = do
 		let dest = localGitDir r </> fromRef ref
-		createDirectoryIfMissing True (takeDirectory dest)
+		createDirectoryIfMissing True (parentDir dest)
 		unlessM (doesFileExist dest) $
 			writeFile dest (fromRef sha)