largefiles: restore standins from non branch-tip parent at rollback correctly Before this patch, "hg rollback" can't restore standins correclty, if: - old parent of the working directory is rollback-ed, and - new parent of the working directory is not branch-tip "overriderollback" uses "merge.update" as a kind of "revert" utility to restore only standins with "node=None", and this makes "merge.update" choose "branch-tip" revision as the updating target unexpectedly. Then, "merge.update" restores standins from the branch-tip revision regardless of the parent of the working directory after rollback and this may cause unexpected behavior. This patch invokes "merge.update" with "node='.'" to restore standins from the parent revision of the working directory. In fact, this "merge.update" invocation will be replaced in the subsequent patch to fix another problem, but this change is usefull to inform reason why such complicated case should be tested.