trailer: split process_input_file into separate pieces

trailer: split process_input_file into separate pieces

Currently, process_input_file does three things:

    (1) parse the input string for trailers,
    (2) print text before the trailers, and
    (3) calculate the position of the input where the trailers end.

Rename this function to parse_trailers(), and make it only do
(1). The caller of this function, process_trailers, becomes responsible
for (2) and (3). These items belong inside process_trailers because they
are both concerned with printing the surrounding text around
trailers (which is already one of the immediate concerns of
process_trailers).

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>