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