largefiles: add lfile argument to updatestandin() for efficiency (API)
Before this patch, updatestandin() takes "standin" argument, and
applies splitstandin() on it to pick out a path to largefile (aka
"lfile" or so) from standin.
But in fact, all callers already knows "lfile". In addition to it,
many callers knows both "standin" and "lfile".
Therefore, making updatestandin() take only one of "standin" or
"lfile" is inefficient.
(binary:application/octet-stream)