Merge branch 'tb/pack-bitmap-traversal-with-boundary'

The object traversal using reachability bitmap done by
"pack-object" has been tweaked to take advantage of the fact that
using "boundary" commits as representative of all the uninteresting
ones can save quite a lot of object enumeration.

* tb/pack-bitmap-traversal-with-boundary:
  pack-bitmap.c: use commit boundary during bitmap traversal
  pack-bitmap.c: extract `fill_in_bitmap()`
  object: add object_array initializer helper function

object.h

@@ -58,6 +58,8 @@ struct object_array {
 
 #define OBJECT_ARRAY_INIT { 0 }
 
+void object_array_init(struct object_array *array);
+
 /*
  * object flag allocation:
  * revision.h:               0---------10         15             23------27