"git interpret-trailers" has been taught a "--parse" and a few
other options to make it easier for scripts to grab existing
trailer lines from a commit log message.

* jk/trailers-parse:
  doc/interpret-trailers: fix "the this" typo
  pretty: support normalization options for %(trailers)
  t4205: refactor %(trailers) tests
  pretty: move trailer formatting to trailer.c
  interpret-trailers: add --parse convenience option
  interpret-trailers: add an option to unfold values
  interpret-trailers: add an option to show only existing trailers
  interpret-trailers: add an option to show only the trailers
  trailer: put process_trailers() options into a struct