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