Branch protection rules prevent first PR of a merge-batch from being merged

[oliver-goetz]

What happened:
When tide is creating a new merge-batch it starts the required tests for the batch, but also trigger those tests for its member with the lowest PR number (PR 9200 in my screenshot below).

tide starts merging the PRs directly when the tests for the merge-batch are completed.
If there are branch-protection rules those rules prevent the PR with the lowest number (PR 9200) from being merged until its individual tests succeeded.
This results in a race between both test runs. When the merge-batch test run wins, all PRs except the first are merged and the first PR becomes part of the next merge-batch.
It is likely that the merge-batch test run wins the race, because it is created first.

As you can see in tide history, poor PR 9200 already lost a couple of races and is still not merged yet.

While I'm writing this issue, 9200 lost another race 😢

What you expected to happen:
I except that all PRs of a merge-batch are merged by tide if the tests succeed.

I could imagine two approaches to solve the issue:

1. tide could wait until all tests of the single PR are finished too before it starts merging
2. tide could still merge directly when the tests of the merge-batch succeed. In this case it could overwrite the contexts of the tests of the single PR

How to reproduce it (as minimally and precisely as possible):

Please provide links to example occurrences, if any:
You could check the history of PR 9200 in our prow instance:
https://prow.gardener.cloud/tide-history?repo=gardener%2Fgardener&pull=9200

Anything else we need to know?:
I started implementing approach 2. in PR #135.