largefiles: restore standins according to restored dirstate Before this patch, standins are restored from the NEW parent of the working directory at "hg rollback", and this causes: - standins removed in the rollback-ed revision are restored, and become orphan, because they are already marked as "R" in the restored dirstate and expected to be unlinked - standins added in the rollback-ed revision are left as they were before rollback, because they are not included in the new parent (this may not be so serious) This patch replaces the "merge.update" invocation with a specific implementation to restore standins according to restored dirstate. This is also the preparation to centralize the logic of updating largefiles into the function wrapping "merge.update" in the subsequent patch. After that patch, "merge.update" will also update largefiles in the working directory and be redundant for restoring standins only.