pseudo-merge: scaffolding for reads

Implement scaffolding within the new pseudo-merge compilation unit
necessary to use the pseudo-merge API from within the pack-bitmap.c
machinery.

The core of this scaffolding is two-fold:

  - The `pseudo_merge` structure itself, which represents an individual
    pseudo-merge bitmap. It has fields for both bitmaps, as well as
    metadata about its position within the memory-mapped region, and
    a few extra bits indicating whether or not it is satisfied, and
    which bitmaps(s, if any) have been read, since they are initialized
    lazily.

  - The `pseudo_merge_map` structure, which holds an array of
    pseudo_merges, as well as a pointer to the memory-mapped region
    containing the pseudo-merge serialization from within a .bitmap
    file.

Note that the `bitmap_index` structure is defined statically within the
pack-bitmap.o compilation unit, so we can't take in a `struct
bitmap_index *`. Instead, wrap the primary components necessary to read
the pseudo-merges in this new structure to avoid exposing the
implementation details of the `bitmap_index` structure.

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

pseudo-merge.c

@@ -454,3 +454,13 @@ void select_pseudo_merges(struct bitmap_writer *writer,
 
 	stop_progress(&progress);
 }
+
+void free_pseudo_merge_map(struct pseudo_merge_map *pm)
+{
+	uint32_t i;
+	for (i = 0; i < pm->nr; i++) {
+		ewah_pool_free(pm->v[i].commits);
+		ewah_pool_free(pm->v[i].bitmap);
+	}
+	free(pm->v);
+}