ref-filter: change parsing function error handling
Continue removing die() calls from ref-filter formatting logic,
so that it could be used by other commands.
Change the signature of parse_ref_filter_atom() by adding
strbuf parameter for error message.
The function returns the position in the used_atom[] array
(as before) for the given atom, or -1 to signal an error.
Upon failure, error message is appended to the strbuf.
Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>