ref-filter: use pretty.c logic for trailers

ref-filter: use pretty.c logic for trailers

Now, ref-filter is using pretty.c logic for setting trailer options.

New to ref-filter:
  :key=<K> - only show trailers with specified key.
  :valueonly[=val] - only show the value part.
  :separator=<SEP> - inserted between trailer lines.
  :key_value_separator=<SEP> - inserted between key and value in trailer lines

Enhancement to existing options(now can take value and its optional):
  :only[=val]
  :unfold[=val]

'val' can be: true, on, yes or false, off, no.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>