Merge branch 'ab/trailers-extra-format'

The "--format=%(trailers)" mechanism gets enhanced to make it
easier to design output for machine consumption.

* ab/trailers-extra-format:
  pretty format %(trailers): add a "key_value_separator"
  pretty format %(trailers): add a "keyonly"
  pretty-format %(trailers): fix broken standalone "valueonly"
  pretty format %(trailers) doc: avoid repetition
  pretty format %(trailers) test: split a long line