Currently Submission Id is constructed by the change-id of the change
that triggered the submission, the timestamp, and a random alphanumeric
string. This is not pretty. Since we recently revealed the submission id
via ChangeInfo in Ic557e0c94 and via queries in I337f7f8ae, it's a good
idea to change the submission id to be more user friendly.
Submission Id is now constructed of "<numeric-id>-<topic>" or
"<numeric-id>" if topic doesn't exist for that change.
The change id is used because it makes the submission id unique (a
change can only be submitted once). The topic is used mostly since this
is currently one of the main use cases for Revert Submission.
Change-Id: Iec8b0db66adf9a7ff2809994acaedef09e4e885c
dd31db99c3