trailer: begin formatting unification

trailer: begin formatting unification

Now that the preparatory refactors are over, we can replace the call to
format_trailers() in interpret-trailers with format_trailer_info(). This
unifies the trailer formatting machinery

In order to avoid breakages in t7502 and t7513, we have to steal the
features present in format_trailers(). Namely, we have to teach
format_trailer_info() as follows:

  (1) make it aware of opts->trim_empty, and

  (2) make it avoid hardcoding ": " as the separator and space (which
  can result in double-printing these characters).

For (2), make it only print the separator and space if we cannot find
any recognized separator somewhere in the key (yes, keys may have a
trailing separator in it --- we will eventually fix this design but not
now). Do so by copying the code out of print_tok_val(), and deleting the
same function.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>