Extract a routine to add all objects whose object ID's first byte is
`cur_fanout` from an existing MIDX.
This function will only be called once, so extracting it is purely
cosmetic to improve the readability of `get_sorted_entries()` (its sole
caller) below.
The functionality is unchanged in this commit.
Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>