optimisation: stream keys through git cat-file --buffer

This is only implemented for git-annex get so far. It makes git-annex
get nearly twice as fast in a repo with 10k files, all of them present!

But, see the TODO for some caveats.

Command/Export.hs

@@ -200,7 +200,7 @@ changeExport r db (PreferredFiltered new) = do
 	mapdiff a oldtreesha newtreesha = do
 		(diff, cleanup) <- inRepo $
 			Git.DiffTree.diffTreeRecursive oldtreesha newtreesha
-		seekActions $ pure $ map a diff
+		sequence_ $ map a diff
 		void $ liftIO cleanup
 
 -- Map of old and new filenames for each changed ExportKey in a diff.