midx-write.c: reduce argument count for `get_sorted_entries()`

midx-write.c: reduce argument count for `get_sorted_entries()`

The function `midx-write.c::get_sorted_entries()` is responsible for
constructing the array of OIDs from a given list of packs which will
comprise the MIDX being written.

The singular call-site for this function looks something like:

    ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr,
                                     &ctx.entries_nr,
                                     ctx.preferred_pack_idx);

This function has five formal arguments, all of which are members of the
shared `struct write_midx_context` used to track various pieces of
information about the MIDX being written.

The function `get_sorted_entries()` dates back to fe1ed56f5e4 (midx:
sort and deduplicate objects from packfiles, 2018-07-12), which came
shortly after 396f257018a (multi-pack-index: read packfile list,
2018-07-12). The latter patch introduced the `pack_list` structure,
which was a precursor to the structure we now know as
`write_midx_context` (c.f. 577dc49696a (midx: rename pack_info to
write_midx_context, 2021-02-18)).

At the time, `get_sorted_entries()` likely could have used the pack_list
structure introduced earlier in 396f257018a, but understandably did not
since the structure only contained three fields (only two of which were
relevant to `get_sorted_entries()`) at the time.

Simplify the declaration of this function by taking a single pointer to
the whole `struct write_midx_context` instead of various members within
it. Since this function is now computing the entire result (populating
both `ctx->entries`, and `ctx->entries_nr`), rename it to something that
doesn't start with "get_" to make clear that this function has a
side-effect.

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