Pass the previous stack to StackUpdate

This starts out like the existing stack, but is not stored in the database
or updated as the stack update proceeds. This allows the StackUpdate object
to store any information that might be relevant to a future rollback.

Change-Id: I94f0dd69b5e7818f811c8c0d50566a79b81acba7
Merge "Pass the previous stack to StackUpdate"