Add possibility to create temporary branch for PR

[R0Wi]

Hey, i was wondering if it's possible to configure the action somehow to first create a new branch in my repository, merge upstream changes in there and then create a new PR from that branch into my base branch?

So basically what the action does today is: create a new PR upstream/head -> fork/base.

If it would do: create a new branch PR-#<NR> in fork, merge upstream/head -> PR-#<NR>, create new PR PR-#<NR> -> fork/base

it would be much easier to for example add commits manually or resolve merge conflicts directly on the temporarily created branch.

If that's a missing feature which you also like to be implemented i'd really be looking forward in creating a draft for this :+1:

[devantler]

Another argument for this is that the current approach does not work with required status checks, as these expect a temporary branch when creating a PR. See the description for the setting.