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