update unicode FilePath handling

Based on http://hackage.haskell.org/trac/ghc/ticket/3307 ,
whether FilePath contains decoded unicode varies by OS.
So, add a configure check for it.

Also, renamed showFile to filePathToString

Command/PreCommit.hs

@@ -33,7 +33,7 @@ perform pair@(file, _) = do
 	ok <- doCommand $ Command.Add.start pair
 	if ok
 		then return $ Just $ cleanup file
-		else error $ "failed to add " ++ showFile file ++ "; canceling commit"
+		else error $ "failed to add " ++ filePathToString file ++ "; canceling commit"
 
 cleanup :: FilePath -> CommandCleanup
 cleanup file = do