Improve performance of Merge request rebasing E2E

What does this MR do and why?

• Replaces uses of ProjectPush with Commit

  • https://docs.gitlab.com/ee/development/testing_guide/end_to_end/best_practices.html#prefer-commit-resource-over-projectpush

• Adds a new has_no_merge_button? to avoid 10 second sleep when checking for the absence of element by using a negatable matcher

  • https://docs.gitlab.com/ee/development/testing_guide/end_to_end/best_practices.html#create-negatable-matchers-to-speed-expect-checks

How to set up and validate locally

1. bundle exec bin/qa Test::Instance::All http://gdk.test:3000 -- qa/specs/features/browser_ui/3_create/merge_request/rebase_merge_request_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.