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.diff_-U_initial..side

@@ -0,0 +1,32 @@
+$ git diff -U initial..side
+diff --git a/dir/sub b/dir/sub
+index 35d242b..7289e35 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++1
++2
+diff --git a/file0 b/file0
+index 01e79c3..f4615da 100644
+--- a/file0
++++ b/file0
+@@ -1,3 +1,6 @@
+ 1
+ 2
+ 3
++A
++B
++C
+diff --git a/file3 b/file3
+new file mode 100644
+index 0000000..7289e35
+--- /dev/null
++++ b/file3
+@@ -0,0 +1,4 @@
++A
++B
++1
++2
+$