rebase: make debug logging more consistent

We emit some lines that mix revision numbers with nodeids, which makes
little sense to me.

hgext/rebase.py

@@ -1198,7 +1198,7 b' def buildstate(repo, dest, rebaseset, co'
                 repo.ui.debug('source is a child of destination\n')
                 return None
 
-        repo.ui.debug('rebase onto %d starting from %s\n' % (dest, root))
+        repo.ui.debug('rebase onto %s starting from %s\n' % (dest, root))
         state.update(dict.fromkeys(rebaseset, revtodo))
         # Rebase tries to turn <dest> into a parent of <root> while
         # preserving the number of parents of rebased changesets:

tests/test-rebase-conflicts.t

@@ -219,7 +219,7 b' Check that the right ancestors is used w'
      summary:     added default.txt
   
   $ hg rebase -s9 -d2 --debug # use debug to really check merge base used
-  rebase onto 2 starting from e31216eec445
+  rebase onto 4bc80088dc6b starting from e31216eec445
   ignoring null merge rebase of 3
   ignoring null merge rebase of 4
   ignoring null merge rebase of 6