list-objects: filter objects in traverse_commit_list

Create traverse_commit_list_filtered() and add filtering
interface to allow certain objects to be omitted from the
traversal.

Update traverse_commit_list() to be a wrapper for the above
with a null filter to minimize the number of callers that
needed to be changed.

Object filtering will be used in a future commit by rev-list
and pack-objects for partial clone and fetch to omit unwanted
objects from the result.

traverse_bitmap_commit_list() does not work with filtering.
If a packfile bitmap is present, it will not be used.  It
should be possible to extend such support in the future (at
least to simple filters that do not require object pathnames),
but that is beyond the scope of this patch series.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.h

@@ -38,6 +38,7 @@ struct object_array {
  * http-push.c:                            16-----19
  * commit.c:                               16-----19
  * sha1_name.c:                                     20
+ * list-objects-filter.c:                             21
  * builtin/fsck.c:  0--3
  */
 #define FLAG_BITS  27