ref-filter: move need_color_reset_at_eol into ref_format

ref-filter: move need_color_reset_at_eol into ref_format

Calling verify_ref_format() doesn't just confirm that the
format is sane; it actually sets some global variables that
will be used later when formatting the refs. These logically
should belong to the ref_format, which would make it
possible to use multiple formats within a single program
invocation.

Let's move one such flag into the ref_format struct. There
are still others that would need to be moved before it would
be safe to use multiple formats, but this commit gives a
blueprint for how that should look.

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