pre-checkout hook

That will fix #112 by checking out into 'llvm-project' directory
(derived from repository name).

containers/buildkite-premerge-debian/Dockerfile

@@ -9,4 +9,5 @@ RUN echo 'install buildkite' ;\
     apt-get clean;
 COPY *.sh /usr/local/bin/
 RUN chmod og+rx /usr/local/bin/*.sh
+COPY --chown=buildkite-agent:buildkite-agent pre-checkout /etc/buildkite-agent/hooks
 CMD ["start_agent.sh"]

containers/buildkite-premerge-debian/pre-checkout

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Copyright 2020 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.
+
+# The `pre-checkout` hook will run just before your pipelines source code is
+# checked out from your SCM provider
+
+set -e
+BUILDKITE_BUILD_CHECKOUT_PATH="${BUILDKITE_BUILD_PATH}/$(echo $BUILDKITE_REPO | sed -E "s#.*/([^/]*)#\1#" | sed "s/.git$//")"