largefiles: introduce "_lfstatuswriters" to customize status reporting "lfutil.getstatuswriter" is the utility to get appropriate function to write largefiles specific status out from "repo._lfstatuswriters". This patch uses "stack" with an element instead of flag like "_isXXXXing" or so, because: - the former works correctly even when customizations are nested, and - ensuring at least one element can ignore empty check