rebase: remove an unused todo

Since we handled skipping obsolete revs when it is relevant, this todo is no
longer useful. We replace the comment with two useful comments.

hgext/rebase.py

@@ -30,8 +30,9 b' import os, errno'
 revtodo = -1
 nullmerge = -2
 revignored = -3
-# To do with obsolescence
+# successor in rebase destination
 revprecursor = -4
+# plain prune (no successor)
 revpruned = -5
 
 cmdtable = {}