Fix commit status API creating branch pipeline even when ref is tag

What does this MR do and why?

Fix commit status API creating branch pipeline even when ref is tag

References

#589315

Screenshots or screen recordings

Before	After
N / A	N / A

How to set up and validate locally

1. Checkout this branch and run gdk locally
2. Create a new personal access token and copy/paste it somewhere to use in step 4
3. Go to any project that has few commits and create a new tag from main branch eg v.2.2.2
4. Call the commits status api to set the status for commit SHA
   • call the POST commits status api eg: http://127.0.0.1:3000/api/v4/projects/PROJECT_ID/statuses/COMMIT_SHA
   • with header PRIVATE-TOKEN set to your newly created personal access token
   • with body { "state": "success", "ref": "v2.2.2" } - make sure tag v2.2.2 exists
5. Opens rails console with rails console command
6. Check the newly created pipeline with pipeline = Ci::Pipeline.order(created_at: :desc).first
7. Notice the tag property is properly set to true now
8. Compare to master branch where it was always set to false