Update remote happens through Gitaly only

This change makes closes another migration! And remotes tests around it,
mostly due to stubbing this was needed. Also, these are tested on the
Gitaly side too:
- https://gitlab.com/gitlab-org/gitaly/issues/791
-
https://gitlab.com/gitlab-org/gitaly/blob/6dd74543f6af8ebcf22aa10e39da004f388fdda5/internal/service/repository/fetch_remote_test.go

Closes https://gitlab.com/gitlab-org/gitaly/issues/789

lib/gitlab/git/remote_mirror.rb

@@ -7,81 +7,8 @@ module Gitlab
       end
 
       def update(only_branches_matching: [])
-        @repository.gitaly_migrate(:remote_update_remote_mirror) do |is_enabled|
-          if is_enabled
-            gitaly_update(only_branches_matching)
-          else
-            rugged_update(only_branches_matching)
-          end
-        end
-      end
-
-      private
-
-      def gitaly_update(only_branches_matching)
-        @repository.gitaly_remote_client.update_remote_mirror(@ref_name, only_branches_matching)
-      end
-
-      def rugged_update(only_branches_matching)
-        local_branches = refs_obj(@repository.local_branches, only_refs_matching: only_branches_matching)
-        remote_branches = refs_obj(@repository.remote_branches(@ref_name), only_refs_matching: only_branches_matching)
-
-        updated_branches = changed_refs(local_branches, remote_branches)
-        push_branches(updated_branches.keys) if updated_branches.present?
-
-        delete_refs(local_branches, remote_branches)
-
-        local_tags = refs_obj(@repository.tags)
-        remote_tags = refs_obj(@repository.remote_tags(@ref_name))
-
-        updated_tags = changed_refs(local_tags, remote_tags)
-        @repository.push_remote_branches(@ref_name, updated_tags.keys) if updated_tags.present?
-
-        delete_refs(local_tags, remote_tags)
-      end
-
-      def refs_obj(refs, only_refs_matching: [])
-        refs.each_with_object({}) do |ref, refs|
-          next if only_refs_matching.present? && !only_refs_matching.include?(ref.name)
-
-          refs[ref.name] = ref
-        end
-      end
-
-      def changed_refs(local_refs, remote_refs)
-        local_refs.select do |ref_name, ref|
-          remote_ref = remote_refs[ref_name]
-
-          remote_ref.nil? || ref.dereferenced_target != remote_ref.dereferenced_target
-        end
-      end
-
-      def push_branches(branches)
-        default_branch, branches = branches.partition do |branch|
-          @repository.root_ref == branch
-        end
-
-        # Push the default branch first so it works fine when remote mirror is empty.
-        branches.unshift(*default_branch)
-
-        @repository.push_remote_branches(@ref_name, branches)
-      end
-
-      def delete_refs(local_refs, remote_refs)
-        refs = refs_to_delete(local_refs, remote_refs)
-
-        @repository.delete_remote_branches(@ref_name, refs.keys) if refs.present?
-      end
-
-      def refs_to_delete(local_refs, remote_refs)
-        default_branch_id = @repository.commit.id
-
-        remote_refs.select do |remote_ref_name, remote_ref|
-          next false if local_refs[remote_ref_name] # skip if branch or tag exist in local repo
-
-          remote_ref_id = remote_ref.dereferenced_target.try(:id)
-
-          remote_ref_id && @repository.rugged_is_ancestor?(remote_ref_id, default_branch_id)
+        @repository.wrapped_gitaly_errors do
+          @repository.gitaly_remote_client.update_remote_mirror(@ref_name, only_branches_matching)
         end
       end
     end