pack-bitmap-write: rename children to reverse_edges

The bitmap_builder_init() method walks the reachable commits in
topological order and constructs a "reverse graph" along the way. At the
moment, this reverse graph contains an edge from commit A to commit B if
and only if A is a parent of B. Thus, the name "children" is appropriate
for for this reverse graph.

In the next change, we will repurpose the reverse graph to not be
directly-adjacent commits in the commit-graph, but instead a more
abstract relationship. The previous changes have already incorporated
the necessary updates to fill_bitmap_commit() that allow these edges to
not be immediate children.

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

pack-bitmap-write.c

@@ -179,7 +179,7 @@ static void compute_xor_offsets(void)
 }
 
 struct bb_commit {
-	struct commit_list *children;
+	struct commit_list *reverse_edges;
 	struct bitmap *bitmap;
 	unsigned selected:1;
 	unsigned idx; /* within selected array */
@@ -228,7 +228,7 @@ static void bitmap_builder_init(struct bitmap_builder *bb,
 
 		for (p = commit->parents; p; p = p->next) {
 			struct bb_commit *ent = bb_data_at(&bb->data, p->item);
-			commit_list_insert(commit, &ent->children);
+			commit_list_insert(commit, &ent->reverse_edges);
 		}
 	}
 }
@@ -358,7 +358,7 @@ void bitmap_writer_build(struct packing_data *to_pack)
 			display_progress(writer.progress, nr_stored);
 		}
 
-		while ((child = pop_commit(&ent->children))) {
+		while ((child = pop_commit(&ent->reverse_edges))) {
 			struct bb_commit *child_ent =
 				bb_data_at(&bb.data, child);