midx.c: extract `midx_fanout_add_pack_fanout()`

midx.c: extract `midx_fanout_add_pack_fanout()`

Extract a routine to add all objects whose object ID's first byte is
`cur_fanout` from a given pack (identified by its index into the `struct
pack_info` array maintained by the MIDX writing routine).

Unlike the previous extraction (for `midx_fanout_add_midx_fanout()`),
this function will be called twice, once for all new packs, and again
for the preferred pack (if it appears in an existing MIDX). The latter
change is to resolve the bug described a few patches ago, and will be
made in the subsequent commit.

Similar to the previous refactoring, this function also enhances the
readability of its caller in `get_sorted_entries()`.

Its functionality is unchanged in this commit.

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