topological index field for commit objects

This is no RFE but rather recurring thoughts whenever I'm working with commit graphs: a topological index attribute for commit objects would be incredible useful. By "topological index" I mean a simple integer for which following condition holds true:

if commit C is part of the history of commit D,
   then C's topological index is smaller than D's index

This would allow topological sorting of commits (e.g. in queues) on the fly and quickly give a "no" answer on the question whether D is part of C's history.

-Marc