- Move the conditions code around, such that the existence of the head
and base is first checked (so a clear error can be given, instead of a
possible server error). This makes it easier to read this code. As the
logic is now grouped together.
- Adds integration testing that simulates the deletion of the base and
head branch and ensures the pull request cannot be opened. The 'normal'
testcase also 'informally' ensures that the previous incorrect condition
is not there, because the branch `base-branch` doesn't exist on the head
repository.
- Resolves #2321
