Fix display of unicode filenames.

Internally, the filenames are stored as un-decoded unicode.
I tried decoding them, but then haskell tries to access the wrong files.
Hmm.

So, I've unhappily chosen option "B", which is to decode filenames before
they are displayed.

Command/PreCommit.hs

@@ -32,7 +32,7 @@ perform pair@(file, _) = do
 	ok <- doCommand $ Command.Add.start pair
 	if ok
 		then return $ Just $ cleanup file
-		else error $ "failed to add " ++ file ++ "; canceling commit"
+		else error $ "failed to add " ++ showFile file ++ "; canceling commit"
 
 cleanup :: FilePath -> CommandCleanup
 cleanup file = do