add post checkout to run git gc

containers/agent-windows-buildkite/Dockerfile

@@ -7,4 +7,5 @@ FROM gcr.io/llvm-premerge-checks/agent-windows-vs2019:${agent_windows_version}
 RUN choco install -y handle --checksum 524E61547C8E26608CDA1B11B6E9471616CCCC48530F6E7EC9131EABF839357E
 COPY start_agent.ps1 C:\scripts\
 COPY pre-checkout.bat c:\buildkite-agent\hooks\
-CMD "powershell C:\scripts\start_agent.ps1"
+COPY post-checkout.bat c:\buildkite-agent\hooks\
+CMD "powershell C:\scripts\start_agent.ps1"

containers/agent-windows-buildkite/post-checkout.bat

@@ -0,0 +1,2 @@
+echo "post-checkout"
+powershell c:\llvm-premerge-checks\scripts\windows\post-checkout.ps1

scripts/windows/post-checkout.ps1

@@ -0,0 +1,17 @@
+# Copyright 2021 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.
+
+echo "running git gc"
+pwd
+git gc