Merge branch 'hv/trailer-formatting'

The logic to handle "trailer" related placeholders in the
"--format=" mechanisms in the "log" family and "for-each-ref"
family is getting unified.

* hv/trailer-formatting:
  ref-filter: use pretty.c logic for trailers
  pretty.c: capture invalid trailer argument
  pretty.c: refactor trailer logic to `format_set_trailers_options()`
  t6300: use function to test trailer options

diff --cc pretty.c
Simple merge

diff --cc ref-filter.c
Simple merge