pack-bitmap: implement `bitmap_writer_has_bitmapped_object_id()`

pack-bitmap: implement `bitmap_writer_has_bitmapped_object_id()`

Prepare to implement pseudo-merge bitmap selection by implementing a
necessary new function, `bitmap_writer_has_bitmapped_object_id()`.

This function returns whether or not the bitmap_writer selected the
given object ID for bitmapping. This will allow the pseudo-merge
machinery to reject candidates for pseudo-merges if they have already
been selected as an ordinary bitmap tip.

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