ref-filter: move is-base tip to used_atom

ref-filter: move is-base tip to used_atom

The string_list "is_base_tips" in struct ref_format stores the
committish part of "is-base:<committish>".  It has the same problems
that its sibling string_list "bases" had.  Fix them the same way as the
previous commit did for the latter, by replacing the string_list with
fields in "used_atom".

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>