Merge branch 'aw/pretty-trailers'
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