Merge branch 'rebase-button-fix' into 'master'

Show rebase button if user can push to source branch

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182793



Merged-by: Phil Hughes <me@iamphill.com>
Approved-by: Chad Lavimoniere <clavimoniere@gitlab.com>
Approved-by: Phil Hughes <me@iamphill.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Annabel Dunstone Gray <annabel.dunstone@gmail.com>