watcher: Avoid loop when adding a file owned by someone else fails in indirect mode because its permissions cannot be modified.

Adding the file moved it to the annex, and then tried to set the mode.
Error unwind then moved the file back, and so the watcher saw the file get
deleted and then added back, and so tried again..

Command/Add.hs

@@ -26,7 +26,6 @@ import qualified Annex.Queue
 #ifndef __ANDROID__
 import Utility.Touch
 #endif
-import Utility.FileMode
 import Config
 import Utility.InodeCache
 import Annex.FileMatcher
@@ -86,11 +85,6 @@ start file = ifAnnexed file addpresent add
  - So a KeySource is returned. Its inodeCache can be used to detect any
  - changes that might be made to the file after it was locked down.
  -
- - In indirect mode, the write bit is removed from the file as part of lock
- - down to guard against further writes, and because objects in the annex
- - have their write bit disabled anyway. This is not done in direct mode,
- - because files there need to remain writable at all times.
- -
  - When possible, the file is hard linked to a temp directory. This guards
  - against some changes, like deletion or overwrite of the file, and
  - allows lsof checks to be done more efficiently when adding a lot of files.
@@ -103,16 +97,28 @@ lockDown file = ifM crippledFileSystem
 	, do
 		tmp <- fromRepo gitAnnexTmpDir
 		createAnnexDirectory tmp
-		unlessM isDirect $ 
-			void $ liftIO $ tryIO $ preventWrite file
-		liftIO $ catchMaybeIO $ do
+		eitherToMaybe <$> tryAnnexIO (go tmp)
+	)
+  where
+	{- In indirect mode, the write bit is removed from the file as part
+	 - of lock down to guard against further writes, and because objects
+	 - in the annex have their write bit disabled anyway.
+	 -
+	 - Freezing the content early also lets us fail early when
+	 - someone else owns the file.
+	 -
+	 - This is not done in direct mode, because files there need to
+	 - remain writable at all times.
+	-}
+  	go tmp = do
+		unlessM isDirect $
+			freezeContent file
+		liftIO $ do
 			(tmpfile, h) <- openTempFile tmp $
 				relatedTemplate $ takeFileName file
 			hClose h
 			nukeFile tmpfile
 			withhardlink tmpfile `catchIO` const nohardlink
-	)
-  where
   	nohardlink = do
 		cache <- genInodeCache file
 		return KeySource