made parentDir return a Maybe FilePath; removed most uses of it

parentDir is less safe than takeDirectory, especially when working
with relative FilePaths. It's really only useful in loops that
want to terminate at /

This commit was sponsored by Audric SCHILTKNECHT.

Annex/ReplaceFile.hs

@@ -46,5 +46,5 @@ replaceFileFrom src dest = go `catchIO` fallback
   where
 	go = moveFile src dest
 	fallback _ = do
-		createDirectoryIfMissing True $ parentDir dest
+		createDirectoryIfMissing True $ takeDirectory dest
 		go