ref-filter: mark unused callback parameters

ref-filter: mark unused callback parameters

The ref-filter code uses virtual functions to handle specific atoms, but
many of the functions ignore some parameters:

  - most atom parsers do not need the ref_format itself, unless they are
    looking at centralized options like use_color, quote_style, etc.

  - meta-atom handlers like append_atom(), align_atom_handler(), etc,
    can't generate errors, so ignore their "err" parameter

  - likewise, the handlers for then/else/end do not even need to look at
    their atom_value, as the "if" handler put everything they need into
    the ref_formatting_state stack

Since these functions all have to conform to virtual function
interfaces, we can't just drop the unused parameters, but must mark them
as UNUSED (to appease -Wunused-parameter).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>