rebase: remove an unused todo

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.

--- a/hgext/rebase.py	Wed Nov 18 13:44:29 2015 -0800
+++ b/hgext/rebase.py	Wed Nov 18 16:06:00 2015 -0800
@@ -30,8 +30,9 @@
 revtodo = -1
 nullmerge = -2
 revignored = -3
-# To do with obsolescence
+# successor in rebase destination
 revprecursor = -4
+# plain prune (no successor)
 revpruned = -5
 
 cmdtable = {}