Push to target branch should rebuild merge requests

## Feature request
I have a multi branch pipeline project and want to build all branches if someone pushes to the target branch.

The story behind it:
In branch driven development for each new feature, bugfix and so on a new branch is created. For a finished branch a merge request is created and it can't be merged until the Jenkins has completed the build. So there are two conditions before merging, manual review and successful Jenkins build.

After merging one branch it should be enforced to rebuild all other branches having the same target branch, f.e. feature-1 and feature-2 should be merged into develop. feature-1 is merged and feature-2 should be automatically rebuild to ensure that it is compatible to the new changes and would not break after merge the build of develop. During the rebuild, the develop is merged into the feature,