difftool--helper: exit when reading a prompt answer fails

An attempt to quit difftool by hitting Ctrl-D (EOF) at its prompt does
not quit it, but is treated as if 'yes' was answered to the prompt and
all following prompts, which is contrary to the user's intent. Fix the
error check.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -49,7 +49,8 @@ launch_merge_tool () {
 		else
 			printf "Launch '%s' [Y/n]: " "$merge_tool"
 		fi
-		if read ans && test "$ans" = n
+		read ans || return
+		if test "$ans" = n
 		then
 			return
 		fi