Change the structure of Submission Id

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

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 0fc733a..67ae65c 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6017,6 +6017,10 @@
 The numeric Change-Id of the change that this change reverts.
 |`submission_id`      |optional|
 ID of the submission of this change. Only set if the status is `MERGED`.
+This ID is equal to the numeric ID of the change that triggered the submission.
+If the change that triggered the submission also has a topic, it will be
+"<id>-<topic>" of the change that triggered the submission.
+The callers must not rely on the format of the submission ID.
 |==================================
 
 [[change-input]]