Fix space leak in fsck and drop commands.

The space leak was somehow caused by this line:

	absfiles <- mapM absPath files

I confess, I don't quite understand why this caused bad buffering,
but apparently the whole pipeline from git-ls-files backed up at that
point.

Happily, rewriting the code to only get the cwd once and use a pure
function to calculate absfiles clears it up, and should be a little more
efficient in syscalls too.

Command.hs

@@ -132,8 +132,7 @@ withAttrFilesInGit :: String -> CommandSeekAttrFiles
 withAttrFilesInGit attr a params = do
 	repo <- Annex.gitRepo
 	files <- liftIO $ runPreserveOrder (Git.inRepo repo) params
-	files' <- filterFiles files
-	liftM (map a) $ liftIO $ Git.checkAttr repo attr files'
+	liftM (map a) $ liftIO $ Git.checkAttr repo attr files
 withBackendFilesInGit :: CommandSeekBackendFiles
 withBackendFilesInGit a params = do
 	repo <- Annex.gitRepo