plumb MeterUpdate into getKey

No behavior changes, but this shows everywhere that a progress meter
could be displayed when hashing a file to add to the annex.

Many of the places don't make sense to display a progress meter though,
eg when importing the copy of the file probably swamps the hashing of
the file.

Command/AddUrl.hs

@@ -372,7 +372,7 @@ finishDownloadWith tmp u url file = do
 		, contentLocation = tmp
 		, inodeCache = Nothing
 		}
-	genKey source backend >>= \case
+	genKey source nullMeterUpdate backend >>= \case
 		Nothing -> return Nothing
 		Just (key, _) -> do
 			addWorkTree u url file key (Just tmp)