In change I7fffa4d34 I added a submission id to all enclosed changes.
However, it seems that if two changes were stacked and submitted,
they would have different submission ids.
Here I ensured that enclosed changes called in ReceiveCommits would
get the same submission id.
I originally thought I also need to update ChangeInserter and
ConsistencyChecker, but it seems they don't have that problem.
Change-Id: I7b49fed64ec26957becc8380af7d43fdf4f50d1c
19d0c1a335