don't unapprove for simple rebase/squash of source branch

With a simple rebase or squash, the actual reviewed content of the pull-request has not changed. The unapprove should not trigger in these cases, despite the history of the source branch being rewritten, since the actual diff of the pull-request remains identical.

Background: users of my add-on (bit-booster) have been reporting bugs to me because their pull-requests get auto-unapproved after they click on the bit-booster rebase button.