status: Fixed to run in nearly constant space.

Before, it leaked space due to caching lists of keys. Now all necessary
data about keys is calculated as they stream in.

The "nearly constant" is due to getKeysPresent, which builds up a lot
of [] thunks as it traverses .git/annex/objects/. Will deal with it later.

Annex/Content.hs

@@ -300,7 +300,7 @@ getKeysPresent' dir = do
 			-- 2 levels of hashing
 			levela <- dirContents dir
 			levelb <- mapM dirContents levela
-			contents <- mapM dirContents (concat levelb)
+			contents <- unsafeInterleaveIO $ mapM dirContents (concat levelb)
 			let files = concat contents
 			return $ mapMaybe (fileKey . takeFileName) files