Make legacy version of the commit-msg hook available

Version 2.16 of gerrit introduced a rewritten commit-msg hook
using the git-interpret-trailers command. That command is only
available in git version 2.2 and later. Further, in 2.16.18 the
hook is updated to use the --where option which is only available
in git version 2.15 and later.

In some environments it is not easy to upgrade the git version to
a later one, i.e. in corporate IT environments where package
versions might be quite old, or when using distributions that do
not have latest versions available.

Make the older version of the hook from gerrit 2.15.x, which does
not use git-intepret-trailers, available from the site so that
users who have an older client can download it.

How to do this will be mentioned in the release notes.

Change-Id: I86d1a84df242954c6568a6c678343507f0265ba7

resources/com/google/gerrit/server/tools/root/TOC

@@ -5,4 +5,6 @@
 
 755 hooks/commit-msg
 
+755 hooks/commit-msg-legacy
+
 755 scripts/reposize.sh

resources/com/google/gerrit/server/tools/root/hooks/commit-msg-legacy

@@ -0,0 +1,193 @@
+#!/bin/sh
+#
+# 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.
+#
+
+unset GREP_OPTIONS
+
+CHANGE_ID_AFTER="Bug|Depends-On|Issue|Test|Feature|Fixes|Fixed"
+MSG="$1"
+
+# Check for, and add if missing, a unique Change-Id
+#
+add_ChangeId() {
+	clean_message=`sed -e '
+		/^diff --git .*/{
+			s///
+			q
+		}
+		/^Signed-off-by:/d
+		/^#/d
+	' "$MSG" | git stripspace`
+	if test -z "$clean_message"
+	then
+		return
+	fi
+
+	# Do not add Change-Id to temp commits
+	if echo "$clean_message" | head -1 | grep -q '^\(fixup\|squash\)!'
+	then
+		return
+	fi
+
+	if test "false" = "`git config --bool --get gerrit.createChangeId`"
+	then
+		return
+	fi
+
+	# Does Change-Id: already exist? if so, exit (no change).
+	if grep -i '^Change-Id:' "$MSG" >/dev/null
+	then
+		return
+	fi
+
+	id=`_gen_ChangeId`
+	T="$MSG.tmp.$$"
+	AWK=awk
+	if [ -x /usr/xpg4/bin/awk ]; then
+		# Solaris AWK is just too broken
+		AWK=/usr/xpg4/bin/awk
+	fi
+
+	# Get core.commentChar from git config or use default symbol
+	commentChar=`git config --get core.commentChar`
+	commentChar=${commentChar:-#}
+
+	# How this works:
+	# - parse the commit message as (textLine+ blankLine*)*
+	# - assume textLine+ to be a footer until proven otherwise
+	# - exception: the first block is not footer (as it is the title)
+	# - read textLine+ into a variable
+	# - then count blankLines
+	# - once the next textLine appears, print textLine+ blankLine* as these
+	#   aren't footer
+	# - in END, the last textLine+ block is available for footer parsing
+	$AWK '
+	BEGIN {
+		if (match(ENVIRON["OS"], "Windows")) {
+			RS="\r?\n" # Required on recent Cygwin
+		}
+		# while we start with the assumption that textLine+
+		# is a footer, the first block is not.
+		isFooter = 0
+		footerComment = 0
+		blankLines = 0
+	}
+
+	# Skip lines starting with commentChar without any spaces before it.
+	/^'"$commentChar"'/ { next }
+
+	# Skip the line starting with the diff command and everything after it,
+	# up to the end of the file, assuming it is only patch data.
+	# If more than one line before the diff was empty, strip all but one.
+	/^diff --git / {
+		blankLines = 0
+		while (getline) { }
+		next
+	}
+
+	# Count blank lines outside footer comments
+	/^$/ && (footerComment == 0) {
+		blankLines++
+		next
+	}
+
+	# Catch footer comment
+	/^\[[a-zA-Z0-9-]+:/ && (isFooter == 1) {
+		footerComment = 1
+	}
+
+	/]$/ && (footerComment == 1) {
+		footerComment = 2
+	}
+
+	# We have a non-blank line after blank lines. Handle this.
+	(blankLines > 0) {
+		print lines
+		for (i = 0; i < blankLines; i++) {
+			print ""
+		}
+
+		lines = ""
+		blankLines = 0
+		isFooter = 1
+		footerComment = 0
+	}
+
+	# Detect that the current block is not the footer
+	(footerComment == 0) && (!/^\[?[a-zA-Z0-9-]+:/ || /^[a-zA-Z0-9-]+:\/\//) {
+		isFooter = 0
+	}
+
+	{
+		# We need this information about the current last comment line
+		if (footerComment == 2) {
+			footerComment = 0
+		}
+		if (lines != "") {
+			lines = lines "\n";
+		}
+		lines = lines $0
+	}
+
+	# Footer handling:
+	# If the last block is considered a footer, splice in the Change-Id at the
+	# right place.
+	# Look for the right place to inject Change-Id by considering
+	# CHANGE_ID_AFTER. Keys listed in it (case insensitive) come first,
+	# then Change-Id, then everything else (eg. Signed-off-by:).
+	#
+	# Otherwise just print the last block, a new line and the Change-Id as a
+	# block of its own.
+	END {
+		unprinted = 1
+		if (isFooter == 0) {
+			print lines "\n"
+			lines = ""
+		}
+		changeIdAfter = "^(" tolower("'"$CHANGE_ID_AFTER"'") "):"
+		numlines = split(lines, footer, "\n")
+		for (line = 1; line <= numlines; line++) {
+			if (unprinted && match(tolower(footer[line]), changeIdAfter) != 1) {
+				unprinted = 0
+				print "Change-Id: I'"$id"'"
+			}
+			print footer[line]
+		}
+		if (unprinted) {
+			print "Change-Id: I'"$id"'"
+		}
+	}' "$MSG" > "$T" && mv "$T" "$MSG" || rm -f "$T"
+}
+_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