Disallow using error-prone merge strategies

DO NOT MERGE / RESTRICT AUTOMERGE and its cousin DO NOT MERGE
ANYWHERE are almost always misused. Make it an error to upload
commits with those merge strategies.

Bug: 156896646
Test: repo upload -n (with DNM and RM commits)
Change-Id: Iae3540fa35dbd7b87a14b53a20be0cd23a206f3a

PREUPLOAD.cfg

@@ -1,3 +1,6 @@
 [Builtin Hooks]
 gofmt = true
 bpfmt = true
+
+[Hook Scripts]
+do_not_use_DO_NOT_MERGE = ${REPO_ROOT}/build/soong/scripts/check_do_not_merge.sh ${PREUPLOAD_COMMIT}

scripts/check_do_not_merge.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+# Copyright (C) 2021 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# 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.
+
+if git show -s --format=%s $1 | grep -qE '(DO NOT MERGE)|(RESTRICT AUTOMERGE)'; then
+    cat >&2 <<EOF
+DO NOT MERGE and RESTRICT AUTOMERGE very often lead to unintended results
+and are not allowed to be used in this project.
+Please use the Merged-In tag to be more explicit about where this change
+should merge to. Google-internal documentation exists at go/merged-in
+
+If this check is mis-triggering or you know Merged-In is incorrect in this
+situation you can bypass this check with \`repo upload --no-verify\`.
+EOF
+    exit 1
+fi