pack-bitmap: make `bitmap_writer_push_bitmapped_commit()` public

pack-bitmap: make `bitmap_writer_push_bitmapped_commit()` public

The pseudo-merge selection code will be added in a subsequent commit,
and will need a way to push the allocated commit structures into the
bitmap writer from a separate compilation unit.

Make the `bitmap_writer_push_bitmapped_commit()` function part of the
pack-bitmap.h header in order to make this possible.

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