Add commit-msg hook from Gerrit Code Review

Currently the hook is unused but later we will invoke it from the main
commit-msg hook.

NOTICE

@@ -3,3 +3,6 @@ Copyright 2010 Kitware, Inc.
 
 This product includes software developed at Kitware, Inc.
 (http://www.kitware.com/).
+
+Portions of this software were developed as part of Gerrit Code Review
+(http://code.google.com/p/gerrit/) by The Android Open Source Project.

gerrit/commit-msg

+#!/bin/sh
+# From Gerrit Code Review 2.1.5
+#
+# Part of Gerrit Code Review (http://code.google.com/p/gerrit/)
+#
+# 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.
+#
+
+CHANGE_ID_AFTER="Bug|Issue"
+MSG="$1"
+
+# Check for, and add if missing, a unique Change-Id
+#
+add_ChangeId() {
+	clean_message=$(sed -e '
+		/^diff --git a\/.*/{
+			s///
+			q
+		}
+		/^Signed-off-by:/d
+		/^#/d
+	' "$MSG" | git stripspace)
+	if test -z "$clean_message"
+	then
+		return
+	fi
+
+	if grep -i '^Change-Id:' "$MSG" >/dev/null
+	then
+		return
+	fi
+
+	id=$(_gen_ChangeId)
+	perl -e '
+		$MSG = shift;
+		$id = shift;
+		$CHANGE_ID_AFTER = shift;
+
+		undef $/;
+		open(I, $MSG); $_ = <I>; close I;
+		s|^diff --git a/.*||ms;
+		s|^#.*$||mg;
+		exit unless $_;
+
+		@message = split /\n/;
+		$haveFooter = 0;
+		$startFooter = @message;
+		for($line = @message - 1; $line >= 0; $line--) {
+			$_ = $message[$line];
+
+			if (/^[a-zA-Z0-9-]+:/ && !m,^[a-z0-9-]+://,) {
+				$haveFooter++;
+				next;
+			}
+			next if /^[ []/;
+			$startFooter = $line if ($haveFooter && /^\r?$/);
+			last;
+		}
+
+		@footer = @message[$startFooter+1..@message];
+		@message = @message[0..$startFooter];
+		push(@footer, "") unless @footer;
+
+		for ($line = 0; $line < @footer; $line++) {
+			$_ = $footer[$line];
+			next if /^($CHANGE_ID_AFTER):/i;
+			last;
+		}
+		splice(@footer, $line, 0, "Change-Id: I$id");
+
+		$_ = join("\n", @message, @footer);
+		open(O, ">$MSG"); print O; close O;
+	' "$MSG" "$id" "$CHANGE_ID_AFTER"
+}
+_gen_ChangeIdInput() {
+	echo "tree $(git write-tree)"
+	if parent=$(git rev-parse HEAD^0 2>/dev/null)
+	then
+		echo "parent $parent"
+	fi
+	echo "author $(git var GIT_AUTHOR_IDENT)"
+	echo "committer $(git var GIT_COMMITTER_IDENT)"
+	echo
+	printf '%s' "$clean_message"
+}
+_gen_ChangeId() {
+	_gen_ChangeIdInput |
+	git hash-object -t commit --stdin
+}
+
+
+add_ChangeId