backport: Check for updated patch when merging
A new patch could be uploaded in between backport time and voting time,
or in between voting time and merge time when tests are being run.
- Checks whether a change has a new revision during
_wait_for_changes_to_be_merged.
- Prompts user to continue or cancel if a new revision is found.
- Updates gerrit request for change details to be able to get all revisions
Bug: T333241