The previous script wrote a lot of change_id on the migration commit
message, and that was unreadable. Developers needed to change the
message by hands for showing the latest change_id for each migrated
module. That was boring.
This patch makes the script write the latest change_id like
* run_tempest.sh: I55311cff4dea1f6c5adaa2a6d51eeee2f6cc71ea
* run_tests.sh: I55311cff4dea1f6c5adaa2a6d51eeee2f6cc71ea
Change-Id: I7ce8e7b911c97ad7757a886f781341cce74c5307
2ce9a6f398