PNW-2019 - Fix rebase to make it correctly on stack change use case

[dfdez]

ℹ️ What's this PR do?

• Update rebase branch to don't create conflicts when changes are made

👀 Where should the reviewer start?

• packages/netlify-cms-backend-github/src/API.ts

📱 How should this be tested?

As reviewer, remember you always have to test the code locally on your machine. To do so follow these instructions:

1. Checkout the PR branch
2. Start the landing you want to test by running npm run dev <landing-name>
3. You will have the site available at http://localhost:8000

Do you need to start multiple landings affected in a git diff?

1. Checkout the PR branch
2. Find the commit SHA you want to diff from
3. Start the landings affected in a git diff with commit SHA like this: npm run dev -- --diff=<COMMIT_SHA>
4. Check out this confluence page for more details about diff in monorepo here

To test this we should:

1. Create a test branch to reproduce the use case.
2. Update the main branch on the CMS to the created branch

Now that we have the basic setup let's reproduce the use case, in order to reproduce this issue follow this steps:

1. Create a stack change
2. Make a change in a collection.
3. Update is made on master
4. Another change is introduced to the same collection that was altered in step 2.

• Test that with feature/PNW-2019_rebase-fix the rebase is not beign made with the main branch