midx-write.c: don't use pack_perm when assigning bitmap_pos

In midx_pack_order(), we compute for each bitmapped pack the first bit
to correspond to an object in that pack, along with how many bits were
assigned to object(s) in that pack.

Initially, each bitmap_nr value is set to zero, and each bitmap_pos
value is set to the sentinel BITMAP_POS_UNKNOWN. This is done to ensure
that there are no packs who have an unknown bit position but a somehow
non-zero number of objects (cf. `write_midx_bitmapped_packs()` in
midx-write.c).

Once the pack order is fully determined, midx_pack_order() sets the
bitmap_pos field for any bitmapped packs to zero if they are still
listed as BITMAP_POS_UNKNOWN.

However, we enumerate the bitmapped packs in order of `ctx->pack_perm`.
This is fine for existing cases, since the only time the
`ctx->pack_perm` array holds a value outside of the addressable range of
`ctx->info` is when there are expired packs, which only occurs via 'git
multi-pack-index expire', which does not support writing MIDX bitmaps.
As a result, the range of ctx->pack_perm covers all values in [0,
`ctx->nr`), so enumerating in this order isn't an issue.

A future change necessary for compaction will complicate this further by
introducing a wrapper around the `ctx->pack_perm` array, which turns the
given `pack_int_id` into one that is relative to the lower end of the
compaction range. As a result, indexing into `ctx->pack_perm` through
this helper, say, with "0" will produce a crash when the lower end of
the compaction range has >0 pack(s) in its base layer, since the
subtraction will wrap around the 32-bit unsigned range, resulting in an
uninitialized read.

But the process is completely unnecessary in the first place: we are
enumerating all values of `ctx->info`, and there is no reason to process
them in a different order than they appear in memory. Index `ctx->info`
directly to reflect that.

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

midx-write.c

@@ -637,7 +637,7 @@ static uint32_t *midx_pack_order(struct write_midx_context *ctx)
 		pack_order[i] = data[i].nr;
 	}
 	for (i = 0; i < ctx->nr; i++) {
-		struct pack_info *pack = &ctx->info[ctx->pack_perm[i]];
+		struct pack_info *pack = &ctx->info[i];
 		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
 			pack->bitmap_pos = 0;
 	}