Separate object listing routines out of rev-list

Create a separate file, list-objects.c, and move object listing
routines from rev-list to it.  The next round will use it in
pack-objects directly.

Signed-off-by: Junio C Hamano <junkio@cox.net>

list-objects.h

@@ -0,0 +1,8 @@
+#ifndef LIST_OBJECTS_H
+#define LIST_OBJECTS_H
+
+void traverse_commit_list(struct rev_info *revs,
+			  void (*show_commit)(struct commit *),
+			  void (*show_object)(struct object_array_entry *));
+
+#endif