Add commit-msg hook

This is the commit-msg hook from:
https://gerrit-review.googlesource.com/tools/hooks/commit-msg

I'm including it here since there should just be a single place that
manages all of our git hooks.

Bug: b/187094215
Change-Id: I60ef07b843fcdbaa944182d5c1611bb4b073a788

commit-msg

diff --git a/commit-msg b/commit-msg
new file mode 100755
index 0000000..a9b4e91
--- /dev/null
+++ b/commit-msg
@@ -0,0 +1,74 @@
+#!/bin/sh
+# From Gerrit Code Review 3.4.0-550-gc960fc8f7d
+#
+# Part of Gerrit Code Review (https://www.gerritcodereview.com/)
+#
+# Copyright (C) 2009 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.
+
+# avoid [[ which is not POSIX sh.
+if test "$#" != 1 ; then
+  echo "$0 requires an argument."
+  exit 1
+fi
+
+if test ! -f "$1" ; then
+  echo "file does not exist: $1"
+  exit 1
+fi
+
+# Do not create a change id if requested
+if test "false" = "`git config --bool --get gerrit.createChangeId`" ; then
+  exit 0
+fi
+
+# $RANDOM will be undefined if not using bash, so don't use set -u
+random=$( (whoami ; hostname ; date; cat $1 ; echo $RANDOM) | git hash-object --stdin)
+dest="$1.tmp.${random}"
+
+trap 'rm -f "${dest}"' EXIT
+
+if ! git stripspace --strip-comments < "$1" > "${dest}" ; then
+   echo "cannot strip comments from $1"
+   exit 1
+fi
+
+if test ! -s "${dest}" ; then
+  echo "file is empty: $1"
+  exit 1
+fi
+
+reviewurl="$(git config --get gerrit.reviewUrl)"
+if test -n "${reviewurl}" ; then
+  if ! git interpret-trailers --parse < "$1" | grep -q '^Link:.*/id/I[0-9a-f]\{40\}$' ; then
+    if ! git interpret-trailers \
+          --trailer "Link: ${reviewurl%/}/id/I${random}" < "$1" > "${dest}" ; then
+      echo "cannot insert link footer in $1"
+      exit 1
+    fi
+  fi
+else
+  # Avoid the --in-place option which only appeared in Git 2.8
+  # Avoid the --if-exists option which only appeared in Git 2.15
+  if ! git -c trailer.ifexists=doNothing interpret-trailers \
+        --trailer "Change-Id: I${random}" < "$1" > "${dest}" ; then
+    echo "cannot insert change-id line in $1"
+    exit 1
+  fi
+fi
+
+if ! mv "${dest}" "$1" ; then
+  echo "cannot mv ${dest} to $1"
+  exit 1
+fi