Avoid loading blob data in snippet update service

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab/blob/master/app/services/snippets/update_service.rb#L101 we retrieve the existing blob name in order to pass it to the repository multi_action method, and also to determine whether it's a create or update operation.

Nevertheless, for that, we load the blob data as well in the call we make. We can leverage this and use Snippet#file_name_on_repo that only retrieves the file name without any blob data.

Does this MR meet the acceptance criteria?

Conformity

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