pack-bitmap-write.c: instrument number of reused bitmaps

When debugging bitmap generation performance, it is useful to know how
many bitmaps were generated from scratch, and how many were the result
of permuting the bit-order of an existing bitmap.

Keep track of the latter, and emit the count as a trace2_data line to
aid in debugging.

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

pack-bitmap-write.c

@@ -384,6 +384,8 @@ static int fill_bitmap_tree(struct bitmap *bitmap,
 	return 0;
 }
 
+static int reused_bitmaps_nr;
+
 static int fill_bitmap_commit(struct bb_commit *ent,
 			      struct commit *commit,
 			      struct prio_queue *queue,
@@ -409,8 +411,10 @@ static int fill_bitmap_commit(struct bb_commit *ent,
 			 * bitmap and add its bits to this one. No need to walk
 			 * parents or the tree for this commit.
 			 */
-			if (old && !rebuild_bitmap(mapping, old, ent->bitmap))
+			if (old && !rebuild_bitmap(mapping, old, ent->bitmap)) {
+				reused_bitmaps_nr++;
 				continue;
+			}
 		}
 
 		/*
@@ -526,6 +530,8 @@ int bitmap_writer_build(struct packing_data *to_pack)
 
 	trace2_region_leave("pack-bitmap-write", "building_bitmaps_total",
 			    the_repository);
+	trace2_data_intmax("pack-bitmap-write", the_repository,
+			   "building_bitmaps_reused", reused_bitmaps_nr);
 
 	stop_progress(&writer.progress);