newvers.sh: fix git false positive -dirty tag

Assuming that any output from `git diff-index --name-only` implies
changes in the working tree results in false positives: files with
metadata, but not content, changes are also listed.

Check that content differences exist before adding the -dirty tag to
the git hash.

PR:		229230
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D15968

sys/conf/newvers.sh

@@ -76,6 +76,35 @@ findvcs()
 	return 1
 }
 
+git_tree_modified()
+{
+	# git diff-index lists both files that are known to have changes as
+	# well as those with metadata that does not match what is recorded in
+	# git's internal state.  The latter case is indicated by an all-zero
+	# destination file hash.
+
+	local fifo vcstop_abs
+
+	fifo=$(mktemp -u)
+	mkfifo -m 600 $fifo
+	vcstop_abs=$(realpath $VCSTOP)
+	$git_cmd --work-tree=${VCSTOP} diff-index HEAD > $fifo &
+	while read smode dmode ssha dsha status file; do
+		if ! expr $dsha : '^00*$' >/dev/null; then
+			rm $fifo
+			return 0
+		fi
+		if ! $git_cmd diff --quiet -- "${vcstop_abs}/${file}"; then
+			rm $fifo
+			return 0
+		fi
+	done < $fifo
+	# No files with content differences.
+	rm $fifo
+	return 1
+}
+
+
 if [ -z "${SYSDIR}" ]; then
     SYSDIR=$(dirname $0)/..
 fi
@@ -240,8 +269,7 @@ if [ -n "$git_cmd" ] ; then
 	if [ -n "$git_b" ] ; then
 		git="${git}(${git_b})"
 	fi
-	if $git_cmd --work-tree=${VCSTOP} diff-index \
-	    --name-only HEAD | read dummy; then
+	if git_tree_modified; then
 		git="${git}-dirty"
 		modified=true
 	fi