automatically launch git repository repair

Added a RemoteChecker thread, that waits for problems to be reported with
remotes, and checks if their git repository is in need of repair.

Currently, only failures to sync with the remote cause a problem to be
reported. This seems enough, but we'll see.

Plugging in a removable drive with a repository on it that is corrupted
does automatically repair the repository, as long as the corruption causes
git push or git pull to fail. Some types of corruption do not, eg
missing/corrupt objects for blobs that git push doesn't need to look at.

So, this is not really a replacement for scheduled git repository fscking.
But it does make the assistant more robust.

This commit is sponsored by Fernando Jimenez.

Assistant/Repair.hs

@@ -33,24 +33,21 @@ import Control.Concurrent.Async
 
 {- When the FsckResults require a repair, tries to do a non-destructive
  - repair. If that fails, pops up an alert. -}
-repairWhenNecessary :: UrlRenderer -> UUID -> Maybe Remote -> FsckResults -> Assistant ()
+repairWhenNecessary :: UrlRenderer -> UUID -> Maybe Remote -> FsckResults -> Assistant Bool
 repairWhenNecessary urlrenderer u mrmt fsckresults
 	| foundBroken fsckresults = do
 		liftAnnex $ writeFsckResults u fsckresults
 		repodesc <- liftAnnex $ Remote.prettyUUID u
-		handle =<< alertDuring (repairingAlert repodesc)
+		ok <- alertWhile (repairingAlert repodesc)
 			(runRepair u mrmt False)
-	| otherwise = noop
-  where
-	handle True = return ()
-	handle False = do
 #ifdef WITH_WEBAPP
-		button <- mkAlertButton True (T.pack "Click Here") urlrenderer $
-			RepairRepositoryR u
-		void $ addAlert $ brokenRepositoryAlert button
-#else
-		return ()
+		unless ok $ do
+			button <- mkAlertButton True (T.pack "Click Here") urlrenderer $
+				RepairRepositoryR u
+			void $ addAlert $ brokenRepositoryAlert button
+		return ok
 #endif
+	| otherwise = return False
 
 runRepair :: UUID -> Maybe Remote -> Bool -> Assistant Bool
 runRepair u mrmt destructiverepair = do