[JENKINS-46028] Look up test result from target branch if building a pull request

[Vlatombe]

When building a pull request for the first time, we don't have any test results. However we can look up the target branch and use its test results as reference.

Updated the demo in #227 to include a gitea server for multibranch support. It can be used to verify the changes by initializing it, waiting for the first build on the main branch to complete, then create a pull request using create-pr.sh.

Relates to #26, #26 (review) probably still stands but feel way more complicated than it needs to be to fulfill the requirement in the context of this plugin.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

Looks about right. Tested interactively I guess? The former PR had some test coverage but I do not remember much.

(There is a demo/ with a multibranch project, but not set up for GitHub or anything else with a “change target head” ~ base branch.)