pending adds now retried for kqueue

Rethought how to keep track of pending adds that need to be retried later.
The commit thread already run up every second when there are changes,
so let's keep pending adds queued as changes until they're safe to add.

Also, the committer is now smarter about avoiding empty commits when
all the adds are currently unsafe, or in the rare case that an add event
for a symlink is not received in time. It may avoid them entirely.

This seems to work as before for inotify, and is untested for kqueue.

(Actually commit batching seems to be improved for inotify, although I'm
not sure why. I'm seeing only two commits made during large batch
operations, and the first of those is the non-batch mode commit.)

Assistant/Watcher.hs

@@ -15,13 +15,14 @@ import Assistant.DaemonStatus
 import Assistant.Changes
 import Utility.DirWatcher
 import Utility.Types.DirWatcher
+import qualified Annex
 import qualified Annex.Queue
 import qualified Git.Command
 import qualified Git.UpdateIndex
 import qualified Git.HashObject
 import qualified Git.LsFiles
 import qualified Backend
-import qualified Annex
+import qualified Command.Add
 import Annex.Content
 import Annex.CatFile
 import Git.Types
@@ -110,22 +111,27 @@ runHandler st dstatus changechan handler file filestatus = void $ do
  - and only one has just closed it. We want to avoid adding a file to the
  - annex that is open for write, to avoid anything being able to change it.
  -
- - We could run lsof on the file here to check for other writer.
- - But, that's slow. Instead, a Change is returned that indicates this file
- - still needs to be added. The committer will handle bundles of these
- - Changes at once.
+ - We could run lsof on the file here to check for other writers.
+ - But, that's slow, and even if there is currently a writer, we will want
+ - to add the file *eventually*. Instead, the file is locked down as a hard
+ - link in a temp directory, with its write bits disabled, for later
+ - checking with lsof, and a Change is returned containing a KeySource
+ - using that hard link. The committer handles running lsof and finishing
+ - the add.
  -}
 onAdd :: Handler
-onAdd file _filestatus dstatus = do
-	ifM (scanComplete <$> getDaemonStatus dstatus)
-		( go
-		, ifM (null <$> inRepo (Git.LsFiles.notInRepo False [file]))
-			( noChange
-			, go
+onAdd file filestatus dstatus
+	| maybe False isRegularFile filestatus = do
+		ifM (scanComplete <$> getDaemonStatus dstatus)
+			( go
+			, ifM (null <$> inRepo (Git.LsFiles.notInRepo False [file]))
+				( noChange
+				, go
+				)
 			)
-		)
+	| otherwise = noChange
 	where
-		go = madeChange file PendingAddChange
+		go = pendingAddChange =<< Command.Add.lockDown 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