Publishing of a changeset does not close a merge request when it has been rebased

The following :

• [draft] (topic1) commit for topic1
• [public] 1st commit

Is pushed to heptapod, the merge request for topic1 is created a reviewed.

When pushing the following :

• [public] commit for topic1
• [public] new commit onto which topic1 has been rebased
• [public] 1st commit

The merge request does not close and has the warning sign "Cannot be merged automatically" in the merge request list. When going to that merge request page, we get a good old 500 error page.

==> /var/log/gitlab/gitlab-rails/production.log <==

ActionView::Template::Error (wrong argument type nil (expected String)):
    20:     -# haml-lint:disable InlineJavaScript
    21:     :javascript
    22:       window.gl = window.gl || {};
    23:       window.gl.mrWidgetData = #{serialize_issuable(@merge_request)}
    24: 
    25:     #js-vue-mr-widget.mr-widget
    26: 
  lib/gitlab/git/repository.rb:464:in `rev_parse_target'
  lib/gitlab/git/repository.rb:458:in `sha_from_ref'
  lib/gitlab/git/repository.rb:489:in `hgsha_from_ref'
  app/models/repository.rb:462:in `method_missing'
  app/models/merge_request_diff.rb:151:in `diff_refs'
  app/models/merge_request.rb:362:in `diff_refs'
  app/models/merge_request.rb:934:in `has_complete_diff_refs?'
  app/services/merge_requests/conflicts/list_service.rb:17:in `can_be_resolved_in_ui?'
  app/presenters/merge_request_presenter.rb:74:in `conflict_resolution_path'
  app/serializers/merge_request_entity.rb:115:in `block in <class:MergeRequestEntity>'
  app/serializers/base_serializer.rb:11:in `represent'
  app/serializers/merge_request_serializer.rb:7:in `represent'
  app/helpers/issuables_helper.rb:42:in `serialize_issuable'
  app/views/projects/merge_requests/show.html.haml:23:in `_app_views_projects_merge_requests_show_html_haml___3793065632001988903_69941037407500'
  app/controllers/projects/merge_requests_controller.rb:79:in `block (3 levels) in show'
  lib/gitlab/gitaly_client.rb:193:in `allow_n_plus_1_calls'
  app/controllers/projects/merge_requests_controller.rb:78:in `block (2 levels) in show'
  app/controllers/projects/merge_requests_controller.rb:62:in `show'
  lib/gitlab/i18n.rb:47:in `with_locale'
  lib/gitlab/i18n.rb:53:in `with_user_locale'
  app/controllers/application_controller.rb:346:in `set_locale'
  lib/gitlab/middleware/multipart.rb:93:in `call'
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
  lib/gitlab/middleware/go.rb:17:in `call'
  lib/gitlab/etag_caching/middleware.rb:11:in `call'
  lib/gitlab/middleware/read_only.rb:30:in `call'
  lib/gitlab/request_context.rb:18:in `call'
  lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'