Approvals are lost on stack rebase with master

[krishna-cf]

Within a stacked PR, all the individual slices need reapproval on every rebase and its currently not scalable.

As part of PR checks we run few tests, so the tests has to be run again. For a stack with 10 slices, I need to ensure all the tests are run and pass with all the approvals present simultaneously. Meanwhile if someone merges to master, then it needs to undergo the same process. Its tiresome and delays the merge.