fix chmod bug (#337)

The implementation of chmod was calculating the new permissions,
but then setting the permissions back to the old ones. This fixes
this bug.

Similarly, the value returned was the od value rather than the new one.

src/Turtle/Prelude.hs

@@ -1253,9 +1253,9 @@ chmod modifyPermissions path = liftIO (do
     let permissions' = fromSystemDirectoryPermissions permissions
     let permissions'' = modifyPermissions permissions'
         changed = permissions' /= permissions''
-    let permissions''' = toSystemDirectoryPermissions permissions'
+    let permissions''' = toSystemDirectoryPermissions permissions''
     when changed (Directory.setPermissions path' permissions''')
-    return permissions' )
+    return permissions'' )
 
 -- | Get a file or directory's user permissions
 getmod :: MonadIO io => FilePath -> io Permissions