Clear merge request error on push to source branch

If a merge error happens for some reason to a merge request, that error will persist even after a new push to the repository. This makes it impossible for a user to merge a merge request. The only workaround is to close the existing merge request and open a new one.

If a user pushes to the source branch, we should clear out the merge error since the diff would likely have changed. If a user pushes to the target branch, we still want to keep the error present so the user can take action. Clearing it out in that case would likely lead to confusion because the error would disappear on unrelated changes.

Closes #34186 (closed)