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/Find.hs

@@ -25,5 +25,5 @@ seek = [withFilesInGit start]
 start :: CommandStartString
 start file = isAnnexed file $ \(key, _) -> do
 	exists <- inAnnex key
-	when exists $ liftIO $ putStrLn $ showFile file
+	when exists $ liftIO $ putStrLn $ filePathToString file
 	return Nothing

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

Command/Unused.hs

@@ -68,7 +68,7 @@ checkUnused = do
 		dropmsg = ["(To remove unwanted data: git-annex dropunused NUMBER)"]
 			
 		table l = ["  NUMBER  KEY"] ++ map cols l
-		cols (n,k) = "  " ++ pad 6 (show n) ++ "  " ++ (showFile . show) k
+		cols (n,k) = "  " ++ pad 6 (show n) ++ "  " ++ (filePathToString . show) k
 		pad n s = s ++ replicate (n - length s) ' '
 
 number :: Int -> [a] -> [(Int, a)]