interpret-trailers: access trailer_info with new helpers
Instead of directly accessing trailer_info members, access them
indirectly through new helper functions exposed by the trailer API.
This is the first of two preparatory commits which will allow us to
use the so-called "pimpl" (pointer to implementation) idiom for the
trailer API, by making the trailer_info struct private to the trailer
implementation (and thus hidden from the API).
Helped-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Linus Arver <linus@ucla.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>