Merge branch 'tb/show-trailers-in-ref-filter'

"git for-each-ref --format=..." learned a new format element,
%(trailers), to show only the commit log trailer part of the log
message.

* tb/show-trailers-in-ref-filter:
ref-filter.c: parse trailers arguments with %(contents) atom
ref-filter.c: use trailer_opts to format trailers
t6300: refactor %(trailers) tests
doc: use "`<literal>`"-style quoting for literal strings
doc: 'trailers' is the preferred way to format trailers
t4205: unfold across multiple lines

diff --cc Documentation/git-for-each-ref.txt
Simple merge

diff --cc ref-filter.c
Simple merge

diff --cc t/t6300-for-each-ref.sh
Simple merge