midx-write.c: pass `start_pack` to `compute_sorted_entries()`

The function `compute_sorted_entries()` is broadly responsible for
building an array of the objects to be written into a MIDX based on the
provided list of packs.

If we have loaded an existing MIDX, however, we may not use all of its
packs, despite loading them into the ctx->info array.

The existing implementation simply skips past the first
ctx->m->num_packs (if ctx->m is non-NULL, indicating that we loaded an
existing MIDX). This is because we read objects in packs from an
existing MIDX via the MIDX itself, rather than from the pack-level
fanout to guarantee a de-duplicated result (see: a40498a1265 (midx: use
existing midx when writing new one, 2018-07-12)).

Future changes (outside the scope of this patch series) to the MIDX code
will require us to skip *at most* that number[^1].

We could tag each pack with a bit that indicates the pack's contents
should be included in the MIDX. But we can just as easily determine the
number of packs to skip by passing in the number of packs we learned
about after processing an existing MIDX.

[^1]: Kind of. The real number will be bounded by the number of packs in
  a MIDX layer, and the number of packs in its base layer(s), but that
  concept hasn't been fully defined yet.

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

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