interpret-trailers: factor trailer rewriting

interpret-trailers: factor trailer rewriting

Extract the trailer rewriting logic into a helper that appends to an
output strbuf.

Update interpret_trailers() to handle file I/O only: read input once,
call the helper, and write the buffered result.

This separation makes it easier to move the helper into trailer.c in the
next commit.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Junio C Hamano <gitster@pobox.com>