Add message to change created via cherry pick

When a new change is created as a result of cherry picking, add
a message on the change:

 "Patch Set <number>: Cherry Picked from branch <name>."

Extend the acceptance test to verify that it works as expected.

Change-Id: I7d210dfa0d04d75e24d03297c36a22fd8f83ee1d
2 files changed