Re-run validation for shiftbot

[Ritwik Sharma]

It will be good to re-run the validation by calling @shiftbot using some command like: @shiftbot rebuild/rerun

I searched for the solution and by reading the code I think we are actually comparing the difference in files every time there is a change in files stored at a specific location. First I thought we are using commit_comment

One solution to our problem is to use pull_request_review_event.

I can't find a command option in https://developer.github.com/webhooks/ as we want.