pretty.c: capture invalid trailer argument

As we would like to use this trailers logic in the ref-filter, it's
nice to get an invalid trailer argument. This will allow us to print
precise error message while using `format_set_trailers_options()` in
ref-filter.

For capturing the invalid argument, we changed the working of
`format_set_trailers_options()` a little bit.
Original logic does "break" and fell through in mainly 2 cases -
    1. unknown/invalid argument
    2. end of the arg string

But now instead of "break", we capture invalid argument and return
non-zero. And non-zero is handled by the caller.
(We prepared the caller to handle non-zero in the previous commit).

Capturing invalid arguments this way will also affects the working
of current logic. As at the end of the arg string it will return non-zero.
So in order to make things correct, introduced an additional conditional
statement i.e if encounter ")", do 'break'.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.h

@@ -151,6 +151,7 @@ int format_set_trailers_options(struct process_trailer_options *opts,
 			struct string_list *filter_list,
 			struct strbuf *sepbuf,
 			struct strbuf *kvsepbuf,
-			const char **arg);
+			const char **arg,
+			char **invalid_arg);
 
 #endif /* PRETTY_H */