Fix handling of multiple GroupSAML identities
A user could have multiple identities with provider: 'group_saml', one
for each GitLab.com group to which they belong. The change in
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14045 would only
update one of these identities via find_or_create_by. To fix this,
also account for saml_provider_id if it's available.
CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30102
Edited by Stan Hu