assistant direct mode file add/change bookkeeping

When a file is changed in direct mode, the old content is probably lost
(at least from the local repo), and bookeeping needs to be updated to
reflect this.

Also, synthetic add events are generated at assistant startup, so
make it detect when the file has not really changed, and avoid re-adding
it.

This does add the overhead of querying the runing git cat-file for the
key that's recorded in git for the file, each time a file is added or
modified in direct mode.

Assistant/Threads/Watcher.hs

@@ -33,6 +33,8 @@ import qualified Git.HashObject
 import qualified Git.LsFiles as LsFiles
 import qualified Backend
 import Annex.Content
+import Annex.Direct
+import Annex.Content.Direct
 import Annex.CatFile
 import Git.Types
 import Config
@@ -60,7 +62,7 @@ watchThread :: NamedThread
 watchThread = NamedThread "Watcher" $ do
 	startup <- asIO1 startupScan
 	direct <- liftAnnex isDirect
-	addhook <- hook $ onAdd direct
+	addhook <- hook $ if direct then onAddDirect else onAdd
 	delhook <- hook onDel
 	addsymlinkhook <- hook onAddSymlink
 	deldirhook <- hook onDelDir
@@ -126,12 +128,27 @@ runHandler handler file filestatus = void $ do
 			liftAnnex $ Annex.Queue.flushWhenFull
 			recordChange change
 
-onAdd :: Bool -> Handler
-onAdd isdirect file filestatus
-	| isdirect = pendingAddChange file
+onAdd :: Handler
+onAdd file filestatus
 	| maybe False isRegularFile filestatus = pendingAddChange file
 	| otherwise = noChange
 
+{- In direct mode, add events are received for both new files, and
+ - modified existing files. Or, in some cases, existing files that have not
+ - really been modified. -}
+onAddDirect :: Handler
+onAddDirect file fs = do
+	v <- liftAnnex $ catKey (Ref $ ':':file)
+	case (v, fs) of
+		(Just key, Just filestatus) ->
+			ifM (liftAnnex $ changedFileStatus key filestatus)
+				( noChange
+				, do
+					liftAnnex $ changedDirect key file
+					pendingAddChange file
+				)
+		_ -> pendingAddChange file
+
 {- A symlink might be an arbitrary symlink, which is just added.
  - Or, if it is a git-annex symlink, ensure it points to the content
  - before adding it.