[EE/Test] Use merge request HEAD ref for detached merge request pipelines
What does this MR do?
This MR is for making sure https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25504 's pipeline passes on EE repo.
What are the relevant issue numbers?
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Documentation created/updated via this MR -
Documentation reviewed by technical writer or follow-up review issue created -
Tests added for this feature/bug -
Tested in all supported browsers -
Conforms to the code review guidelines -
Conforms to the merge request performance guidelines -
Conforms to the style guides -
Conforms to the database guides -
Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process. -
EE specific content should be in the top level /eefolder -
For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan? -
Security reports checked/validated by reviewer