pack-bitmap.c: teach `bitmap_for_commit()` about incremental MIDXs

The pack-bitmap machinery uses `bitmap_for_commit()` to locate the
EWAH-compressed bitmap corresponding to some given commit object.

Teach this function about incremental MIDX bitmaps by teaching it to
recur on earlier bitmap layers when it fails to find a given commit in
the current layer.

The changes to do so are as follows:

  - Avoid initializing hash_pos at its declaration, since
    bitmap_for_commit() is now a recursive function and may receive a
    NULL bitmap_index pointer as its first argument.

  - In cases where we would previously return NULL (to indicate that a
    lookup failed and the given bitmap_index does not contain an entry
    corresponding to the given commit), recursively call the function on
    the previous bitmap layer.

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

pack-bitmap.c

@@ -941,18 +941,21 @@ corrupt:
 struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
 				      struct commit *commit)
 {
-	khiter_t hash_pos = kh_get_oid_map(bitmap_git->bitmaps,
+	khiter_t hash_pos;
-					   commit->object.oid);
+	if (!bitmap_git)
+		return NULL;
+
+	hash_pos = kh_get_oid_map(bitmap_git->bitmaps, commit->object.oid);
 	if (hash_pos >= kh_end(bitmap_git->bitmaps)) {
 		struct stored_bitmap *bitmap = NULL;
 		if (!bitmap_git->table_lookup)
-			return NULL;
+			return bitmap_for_commit(bitmap_git->base, commit);
 
 		/* this is a fairly hot codepath - no trace2_region please */
 		/* NEEDSWORK: cache misses aren't recorded */
 		bitmap = lazy_bitmap_for_commit(bitmap_git, commit);
 		if (!bitmap)
-			return NULL;
+			return bitmap_for_commit(bitmap_git->base, commit);
 		return lookup_stored_bitmap(bitmap);
 	}
 	return lookup_stored_bitmap(kh_value(bitmap_git->bitmaps, hash_pos));