Documentation: describe incremental MIDX bitmaps

Prepare to implement support for reachability bitmaps for the new
incremental multi-pack index (MIDX) feature over the following commits.

This commit begins by first describing the relevant format and usage
details for incremental MIDX bitmaps.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/multi-pack-index.adoc

@@ -164,6 +164,78 @@ objects_nr($H2) + objects_nr($H1) + i
 (in the C implementation, this is often computed as `i +
 m->num_objects_in_base`).
 
+=== Pseudo-pack order for incremental MIDXs
+
+The original implementation of multi-pack reachability bitmaps defined
+the pseudo-pack order in linkgit:gitformat-pack[5] (see the section
+titled "multi-pack-index reverse indexes") roughly as follows:
+
+____
+In short, a MIDX's pseudo-pack is the de-duplicated concatenation of
+objects in packs stored by the MIDX, laid out in pack order, and the
+packs arranged in MIDX order (with the preferred pack coming first).
+____
+
+In the incremental MIDX design, we extend this definition to include
+objects from multiple layers of the MIDX chain. The pseudo-pack order
+for incremental MIDXs is determined by concatenating the pseudo-pack
+ordering for each layer of the MIDX chain in order. Formally two objects
+`o1` and `o2` are compared as follows:
+
+1. If `o1` appears in an earlier layer of the MIDX chain than `o2`, then
+  `o1` sorts ahead of `o2`.
+
+2. Otherwise, if `o1` and `o2` appear in the same MIDX layer, and that
+   MIDX layer has no base, then if one of `pack(o1)` and `pack(o2)` is
+   preferred and the other is not, then the preferred one sorts ahead of
+   the non-preferred one. If there is a base layer (i.e. the MIDX layer
+   is not the first layer in the chain), then if `pack(o1)` appears
+   earlier in that MIDX layer's pack order, then `o1` sorts ahead of
+   `o2`. Likewise if `pack(o2)` appears earlier, then the opposite is
+   true.
+
+3. Otherwise, `o1` and `o2` appear in the same pack, and thus in the
+   same MIDX layer. Sort `o1` and `o2` by their offset within their
+   containing packfile.
+
+Note that the preferred pack is a property of the MIDX chain, not the
+individual layers themselves. Fundamentally we could introduce a
+per-layer preferred pack, but this is less relevant now that we can
+perform multi-pack reuse across the set of packs in a MIDX.
+
+=== Reachability bitmaps and incremental MIDXs
+
+Each layer of an incremental MIDX chain may have its objects (and the
+objects from any previous layer in the same MIDX chain) represented in
+its own `*.bitmap` file.
+
+The structure of a `*.bitmap` file belonging to an incremental MIDX
+chain is identical to that of a non-incremental MIDX bitmap, or a
+classic single-pack bitmap. Since objects are added to the end of the
+incremental MIDX's pseudo-pack order (see above), it is possible to
+extend a bitmap when appending to the end of a MIDX chain.
+
+(Note: it is possible likewise to compress a contiguous sequence of MIDX
+incremental layers, and their `*.bitmap` files into a single layer and
+`*.bitmap`, but this is not yet implemented.)
+
+The object positions used are global within the pseudo-pack order, so
+subsequent layers will have, for example, `m->num_objects_in_base`
+number of `0` bits in each of their four type bitmaps. This follows from
+the fact that we only write type bitmap entries for objects present in
+the layer immediately corresponding to the bitmap).
+
+Note also that only the bitmap pertaining to the most recent layer in an
+incremental MIDX chain is used to store reachability information about
+the interesting and uninteresting objects in a reachability query.
+Earlier bitmap layers are only used to look up commit and pseudo-merge
+bitmaps from that layer, as well as the type-level bitmaps for objects
+in that layer.
+
+To simplify the implementation, type-level bitmaps are iterated
+simultaneously, and their results are OR'd together to avoid recursively
+calling internal bitmap functions.
+
 Future Work
 -----------