PR's "Rebase and Merge" should not alter commits if the head branch is already on top of the main one

[castarco]

Let's say that we have the following branches:

main: A->B->C->D
new:  A->B->C->D->E->F->G

It's clear that if we rebased new on top of main, nothing should change, the commits should remain as they are.

Sadly, that's not what happens when we press the PR's "Rebase and Merge" button, the commits are modified, and the attached PGP signatures are lost. This is not how rebases work locally, and neither how they work in other platforms, such as Gitlab.

In order to support more workflows, it would be preferable that Github sticks to more intuitive and common behaviours when it comes to "rebase and merge" pull requests.