Changing target branch of a merge request does not rerun pipeline

Problem to solve

As a developer who needs to change target branch, I want a pipeline to run when that happens, so i don't have to remember to rebase and run the pipeline manually and/or miss bugs I may have introduced.

Steps today to replicate

1. Have a project with MR pipelines
   • I tested with a pipeline which only has jobs which are limited to merge_requests only. I don't know if that is relevant for the bug
2. Open a new merge request which merges branch A into B
3. The pipeline starts and runs trough
4. Now press the edit (pen) button next to the MR title and change the target branch from B to C
5. The pipeline does not rerun

Proposal

Make changing target branch a merge_request_event so that a pipeline can be triggered through workflow like with other merge_request_events.

Workaround

The current workaround to this which is not automated is to rebase the Merge Request w/ CI after changing target branch.