pack-bitmap: reword comments in test_bitmap_commits()

pack-bitmap: reword comments in test_bitmap_commits()

The comment in pack-bitmap.c:test_bitmap_commits(), suggests that
we can avoid reading the commit table altogether. However, this
comment is misleading. The reason we load bitmap entries here is
because test_bitmap_commits() needs to print the commit IDs from the
bitmap, and we must read the bitmap entries to obtain those commit IDs.
So reword this comment.

Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>