pack-bitmap: tag bitmapped packs with their corresponding MIDX

The next commit will need to use the bitmap's MIDX (if one exists) to
translate bit positions into pack-relative positions in the source pack.

Ordinarily, we'd use the "midx" field of the bitmap_index struct. But
since that struct is defined within pack-bitmap.c, and our caller is in
a separate compilation unit, we do not have access to the MIDX field.

Instead, add a "from_midx" field to the bitmapped_pack structure so that
we can use that piece of data from outside of pack-bitmap.c. The caller
that uses this new piece of information will be added in the following
commit.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

midx.c

@@ -496,6 +496,7 @@ int nth_bitmapped_pack(struct repository *r, struct multi_pack_index *m,
 				 MIDX_CHUNK_BITMAPPED_PACKS_WIDTH * local_pack_int_id +
 				 sizeof(uint32_t));
 	bp->pack_int_id = pack_int_id;
+	bp->from_midx = m;
 
 	return 0;
 }

pack-bitmap.c

@@ -2322,6 +2322,7 @@ void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
 		packs[packs_nr].pack_int_id = pack_int_id;
 		packs[packs_nr].bitmap_nr = pack->num_objects;
 		packs[packs_nr].bitmap_pos = 0;
+		packs[packs_nr].from_midx = bitmap_git->midx;
 
 		objects_nr = packs[packs_nr++].bitmap_nr;
 	}

pack-bitmap.h

@@ -60,6 +60,7 @@ struct bitmapped_pack {
 	uint32_t bitmap_pos;
 	uint32_t bitmap_nr;
 
+	struct multi_pack_index *from_midx; /* MIDX only */
 	uint32_t pack_int_id; /* MIDX only */
 };