add git ls-tree --long parser

Not yet used, but allows getting the size of items in the tree fairly
cheaply.

I noticed that CmdLine.Seek uses ls-tree and the feeds the files into
another long-running process to check their size. That would be an
example of a place that might be sped up by using this. Although in that
particular case, it only needs to know the size of unlocked files, not
locked. And since enabling --long probably doubles the ls-tree runtime
or more, the overhead of using it there may outwweigh the benefit.

Annex/WorkTree.hs

@@ -82,7 +82,10 @@ scanUnlockedFiles = whenM (inRepo Git.Ref.headExists <&&> not <$> isBareRepo) $
 	dropold <- liftIO $ newMVar $ 
 		Database.Keys.runWriter $
 			liftIO . Database.Keys.SQL.dropAllAssociatedFiles
-	(l, cleanup) <- inRepo $ Git.LsTree.lsTree Git.LsTree.LsTreeRecursive Git.Ref.headRef
+	(l, cleanup) <- inRepo $ Git.LsTree.lsTree
+		Git.LsTree.LsTreeRecursive
+		(Git.LsTree.LsTreeLong False)
+		Git.Ref.headRef
 	forM_ l $ \i -> 
 		when (isregfile i) $
 			maybe noop (add dropold i)