pretty: add %(trailers) format for displaying trailers of a commit message

pretty: add %(trailers) format for displaying trailers of a commit message

Recent patches have expanded on the trailers.c code and we have the
builtin commant git-interpret-trailers which can be used to add or
modify trailer lines. However, there is no easy way to simply display
the trailers of a commit message.

Add support for %(trailers) format modifier which will use the
trailer_info_get() calls to read trailers in an identical way as git
interpret-trailers does. Use a long format option instead of a short
name so that future work can more easily unify ref-filter and pretty
formats.

Add documentation and tests for the same.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>