midx-write.c: factor fanout layering from `compute_sorted_entries()`

midx-write.c: factor fanout layering from `compute_sorted_entries()`

When computing the set of objects to appear in a MIDX, we use
compute_sorted_entries(), which handles objects from various existing
sources one fanout layer at a time.

The process for computing this set is slightly different during MIDX
compaction, so factor out the existing functionality into its own
routine to prevent `compute_sorted_entries()` from becoming too
difficult to read.

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