webapp: Progess bar fixes for many types of special remotes.

There was confusion in different parts of the progress bar code about
whether an update contained the total number of bytes transferred, or the
number of bytes transferred since the last update. One way this bug
showed up was progress bars that seemed to stick at zero for a long time.
In order to fix it comprehensively, I add a new BytesProcessed data type,
that is explicitly a total quantity of bytes, not a delta.

Note that this doesn't necessarily fix every problem with progress bars.
Particularly, buffering can now cause progress bars to seem to run ahead
of transfers, reaching 100% when data is still being uploaded.

Command/TransferInfo.hs

@@ -13,6 +13,7 @@ import Annex.Content
 import Logs.Transfer
 import Types.Key
 import qualified Fields
+import Utility.Metered
 
 def :: [Command]
 def = [noCommit $ command "transferinfo" paramKey seek SectionPlumbing
@@ -50,10 +51,14 @@ start (k:[]) = do
 			(update, tfile, _) <- mkProgressUpdater t info
 			liftIO $ mapM_ void
 				[ tryIO $ forever $ do
-					bytes <- readish <$> getLine
-					maybe (error "transferinfo protocol error") update bytes
+					bytes <- readUpdate
+					maybe (error "transferinfo protocol error")
+						(update . toBytesProcessed) bytes
 				, tryIO $ removeFile tfile
 				, exitSuccess
 				]
 	stop
 start _ = error "wrong number of parameters"
+
+readUpdate :: IO (Maybe Integer)
+readUpdate = readish <$> getLine