The %(trailers) formatter in "git log --format=..."  now allows to
optionally pick trailers selectively by keyword, show only values,
etc.

* aw/pretty-trailers:
  pretty: add support for separator option in %(trailers)
  strbuf: separate callback for strbuf_expand:ing literals
  pretty: add support for "valueonly" option in %(trailers)
  pretty: allow showing specific trailers
  pretty: single return path in %(trailers) handling
  pretty: allow %(trailers) options with explicit value
  doc: group pretty-format.txt placeholders descriptions