Merge branch 'hv/ref-filter-trailers-atom-parsing-fix'

Merge branch 'hv/ref-filter-trailers-atom-parsing-fix'

The parser for "git for-each-ref --format=..." was too loose when
parsing the "%(trailers...)" atom, and forgot that "trailers" and
"trailers:<modifiers>" are the only two allowed forms, which has
been corrected.

* hv/ref-filter-trailers-atom-parsing-fix:
  ref-filter: 'contents:trailers' show error if `:` is missing
  t6300: unify %(trailers) and %(contents:trailers) tests