[RFE] Use the Mergify config from the pull request destination branch rather than the default branch

[bowlofeggs]

I've observed that Mergify uses the .mergify.yml file from my project's default branch, rather than using the .mergify.yml from the pull request's target branch. It would be nice if it used the policy from the destination branch so that we can keep the rule list simpler. I can achieve what I want by setting different rules for different branches in the default branch's .mergify.yml, but this makes the config more complex there.

[0xc0170]

This would be nice to have.

As docs state: "In the the default repository branch configured on GitHub — usually master.". However as the first stage for enabling mergify to a repository, would be to test it on a branch and creating PR against this branch to test the configuration and defined pull request workflow. There is a simulator but someone would rather test it on PRs to get the config prototype working.