fix too early close of shared lock file

This fixes a reversion introduced in commit
ac56a5c2a0.

I didn't notice there that it was handling the case of a shared lock
file that was still open elsewhere by not running the close action.

This was especially deadly when annex.pidlock is set, as it caused early
deletion of the pid lock file.

Sponsored-by: Dartmouth College's Datalad project

Utility/LockPool/STM.hs

@@ -122,15 +122,15 @@ releaseLock :: LockHandle -> IO ()
 releaseLock h = go =<< atomically (tryTakeTMVar h)
   where
 	go (Just (pool, file, closelockfile)) = do
-		m <- atomically $ do
+		(m, lastuser) <- atomically $ do
 			m <- takeTMVar pool
 			return $ case M.lookup file m of
 				Just (LockStatus mode n)
-					| n == 1 -> (M.delete file m)
+					| n == 1 -> (M.delete file m, True)
 					| otherwise ->
-						(M.insert file (LockStatus mode (pred n)) m)
-				Nothing -> m
-		() <- closelockfile
+						(M.insert file (LockStatus mode (pred n)) m, False)
+				Nothing -> (m, True)
+		() <- when lastuser closelockfile
 		atomically $ putTMVar pool m
 	-- The LockHandle was already closed.
 	go Nothing = return ()