combine PendingAddChanges for the same file into one

In v6 unlocked mode, this fixes a problem that was making eg,
echo > file cause the assistant to copy the file to the annex object,
instead of hard linking it. That because 2 change events were seen
(one for opening the file and one for closing) and processed together
the file was then locked down twice. Which meant it had mutiple hard links,
and so prevented linkAnnex from hard linking it.

There might be scenarios where multiple events come in, but staggered such
that a file gets locked down repeatedly, and it would still be copied to
the annex object in that case.

Assistant/Threads/Committer.hs

@@ -55,7 +55,8 @@ commitThread = namedThread "Committer" $ do
 			=<< annexDelayAdd <$> Annex.getGitConfig
 	msg <- liftAnnex Command.Sync.commitMsg
 	waitChangeTime $ \(changes, time) -> do
-		readychanges <- handleAdds havelsof delayadd changes
+		readychanges <- handleAdds havelsof delayadd $
+			simplifyChanges changes
 		if shouldCommit False time (length readychanges) readychanges
 			then do
 				debug

Assistant/Types/Changes.hs

@@ -1,10 +1,12 @@
 {- git-annex assistant change tracking
  -
- - Copyright 2012-2013 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2015 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE BangPatterns #-}
+
 module Assistant.Types.Changes where
 
 import Types.KeySource
@@ -14,6 +16,7 @@ import Annex.Ingest
 
 import Control.Concurrent.STM
 import Data.Time.Clock
+import qualified Data.Set as S
 
 {- An un-ordered pool of Changes that have been noticed and should be
  - staged and committed. Changes will typically be in order, but ordering
@@ -76,3 +79,22 @@ finishedChange c@(InProcessAddChange {}) k = Change
 	, changeInfo = AddKeyChange k
 	}
 finishedChange c _ = c
+
+{- Combine PendingAddChanges that are for the same file.
+ - Multiple such often get noticed when eg, a file is opened and then
+ - closed in quick succession. -}
+simplifyChanges :: [Change] -> [Change]
+simplifyChanges [c] = [c]
+simplifyChanges cl = go cl S.empty []
+  where
+ 	go [] _ l = reverse l
+	go (c:cs) seen l
+		| isPendingAddChange c =
+			if S.member f seen 
+				then go cs seen l
+				else 
+					let !seen' = S.insert f seen
+					in go cs seen' (c:l)
+		| otherwise = go cs seen (c:l)
+	  where
+		f = changeFile c