diff-parseopt: restore -U (no argument) behavior

Before d473e2e0e8 (diff.c: convert -U|--unified, 2019-01-27), -U and
--unified are implemented with a custom parser opt_arg() in diff.c. I
didn't check this code carefully and not realize that it's the
equivalent of PARSE_OPT_NONEG | PARSE_OPT_OPTARG.

In other words, if -U is specified without any argument, the option
should be accepted, and the default value should be used. Without
PARSE_OPT_OPTARG, parse_options() will reject this case and cause a
regression.

Reported-by: Bryan Turner <bturner@atlassian.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013-diff-various.sh

@@ -330,6 +330,8 @@ format-patch --inline --stdout initial..master^^
 format-patch --stdout --cover-letter -n initial..master^
 
 diff --abbrev initial..side
+diff -U initial..side
+diff -U1 initial..side
 diff -r initial..side
 diff --stat initial..side
 diff -r --stat initial..side