hooks--pre-commit: detect non-ASCII when renaming

When diff.renames is turned on, the diff-filter will not return renamed
files (or copied ones with diff.renames=copy) and potential non-ASCII
characters would not be caught by this hook.

Use the plumbing command diff-index instead of the porcelain one to not
be affected by diff.rename.

Signed-off-by: Julian Prein <druckdev@protonmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-commit.sample

@@ -28,7 +28,7 @@ if [ "$allownonascii" != "true" ] &&
 	# Note that the use of brackets around a tr range is ok here, (it's
 	# even required, for portability to Solaris 10's /usr/bin/tr), since
 	# the square bracket bytes happen to fall in the designated range.
-	test $(git diff --cached --name-only --diff-filter=A -z $against |
+	test $(git diff-index --cached --name-only --diff-filter=A -z $against |
 	  LC_ALL=C tr -d '[ -~]\0' | wc -c) != 0
 then
 	cat <<\EOF