pretty: move trailer formatting to trailer.c

pretty: move trailer formatting to trailer.c

The next commit will add many features to the %(trailer)
placeholder in pretty.c. We'll need to access some internal
functions of trailer.c for that, so our options are either:

  1. expose those functions publicly

or

  2. make an entry point into trailer.c to do the formatting

Doing (2) ends up exposing less surface area, though do note
that caveats in the docstring of the new function.

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