trailer: pass process_trailer_opts to trailer_info_get()

trailer: pass process_trailer_opts to trailer_info_get()

Most of the trailer code has an "opts" struct which is
filled in by the caller. We don't pass it down to
trailer_info_get(), which does the initial parsing, because
there hasn't yet been a need to do so.

Let's start passing it down in preparation for adding new
options. Note that there's a single caller which doesn't
otherwise have such an options struct. Since it's just one
caller (that we'd have to modify anyway), let's not bother
with any special treatment like accepting a NULL options
struct, and just have it allocate one with the defaults.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>