display scanning message whenever reconcileStaged has enough files to chew on

Clear visible progress bar first.

Removed showSideActionAfter because it can't be used in reconcileStaged
(import loop). Instead, it counts the number of files it
processes and displays it after it's seen a sufficient to know it's
taking a while.

Sponsored-by: Dartmouth College's Datalad project

Database/Keys.hs

@@ -7,6 +7,7 @@
 
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE BangPatterns #-}
 
 module Database.Keys (
 	DbHandle,
@@ -403,7 +404,7 @@ reconcileStaged qh = do
 				proct <- liftIO $ async $
 					procthread mdreader catfeeder
 						`finally` void catcloser
-				dbchanged <- dbwriter False catreader
+				dbchanged <- dbwriter False largediff catreader
 				-- Flush database changes now
 				-- so other processes can see them.
 				when dbchanged $
@@ -420,8 +421,27 @@ reconcileStaged qh = do
 			Just _ -> procthread mdreader catfeeder
 			Nothing -> return ()
 
-		dbwriter dbchanged catreader = liftIO catreader >>= \case
+		dbwriter dbchanged n catreader = liftIO catreader >>= \case
 			Just (ka, content) -> do
 				changed <- ka (parseLinkTargetOrPointerLazy =<< content)
-				dbwriter (dbchanged || changed) catreader
+				!n' <- countdownToMessage n
+				dbwriter (dbchanged || changed) n' catreader
 			Nothing -> return dbchanged
+
+	-- When the diff is large, the scan can take a while,
+	-- so let the user know what's going on.
+	countdownToMessage n
+		| n < 1 = return 0
+		| n == 1 = do
+			showSideAction "scanning for annexed files"
+			return 0
+		| otherwise = return (pred n)
+
+	-- How large is large? Too large and there will be a long
+	-- delay before the message is shown; too short and the message
+	-- will clutter things up unncessarily. It's uncommon for 1000
+	-- files to change in the index, and processing that many files
+	-- takes less than half a second, so that seems about right.
+	largediff :: Int
+	largediff = 1000
+