pack-bitmap.c: introduce 'bitmap_is_preferred_refname()'

pack-bitmap.c: introduce 'bitmap_is_preferred_refname()'

In a recent commit, pack-objects learned support for the
'pack.preferBitmapTips' configuration. This patch prepares the
multi-pack bitmap code to respect this configuration, too.

The yet-to-be implemented code will find that it is more efficient to
check whether each reference contains a prefix found in the configured
set of values rather than doing an additional traversal.

Implement a function 'bitmap_is_preferred_refname()' which will perform
that check. Its caller will be added in a subsequent patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>