When 'master'-branch is used as ready-branch, we still try to do post-build steps. 'master'-branch is not deleted, as it is protected also elsewhere, but it is still a logic error.

Relevant build log example:
Checking out integration target branch master and pulling latest changes
[PREINT] git checkout -B master lower-letters/master
[lower-letters] $ git checkout -B master lower-letters/master
[PREINT] git pull lower-letters master
[lower-letters] $ git pull lower-letters master
Preparing to merge changes in commit 987bf3e72df9d0b23fe9e3a0f5a12596a0a206f5 to integration branch master
[PREINT] git merge -m "New data log line

[lower-letters/ready/683513942]

[lower-letters/master]" 987bf3e72df9d0b23fe9e3a0f5a12596a0a206f5 --no-ff
[lower-letters] $ git merge -m "New data log line

[lower-letters/ready/683513942]

[lower-letters/master]" 987bf3e72df9d0b23fe9e3a0f5a12596a0a206f5 --no-ff
Performing pre-verified post build steps
[PREINT-GIT] Using the master branch for polling and development is not allowed since it will attempt to merge it to other branches and delete it after.
[PREINT-GIT] Commiting changes
[PREINT] git push lower-letters master
[lower-letters] $ git push lower-letters master
[PREINT-GIT] Deleting development branch