fetch main in diff script, not in post-checkout

containers/buildkite-linux/Dockerfile

@@ -83,7 +83,6 @@ RUN echo 'install buildkite' ;\
     apt-get clean;
 
 COPY --chown=buildkite-agent:buildkite-agent pre-checkout /etc/buildkite-agent/hooks
-COPY --chown=buildkite-agent:buildkite-agent post-checkout /etc/buildkite-agent/hooks
 
 # buildkite working directory
 VOLUME /var/lib/buildkite-agent

containers/buildkite-linux/post-checkout

@@ -1,23 +0,0 @@
-#!/bin/bash
-# Copyright 2023 Google LLC
-#
-# Licensed under the the Apache License v2.0 with LLVM Exceptions (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     https://llvm.org/LICENSE.txt
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-# See https://buildkite.com/docs/agent/v3/hooks#job-lifecycle-hooks
-# The `post-checkout` hook runs after checkout.
-
-# Pull changes from origin/main so that diff origin/main...HEAD will show a
-# correct diff of Pull Requests that have multiple commits.
-
-set -euo pipefail
-git fetch origin main:main