Rebase button not available for WIP merge requests

Summary

Rebase button not available for WIP merge requests

Steps to reproduce

1. Create a project with fast-forward only policy
2. Create a branch 'test-branch'
3. Make changes to the 'test-branch'
4. Create an MR out of 'test-branch' with name '[WIP] test-MR'
5. Make changes to master
6. Notice that the rebase button is not available in the MR page

Example Project

https://gitlab.com/tpresa/test-wip-rebase/merge_requests/1

What is the current bug behavior?

Rebase button is not available

What is the expected correct behavior?

Rebase button should be available even for WIP

Possible fixes

ZD https://gitlab.zendesk.com/agent/tickets/91478