rebase: fix 'rebase onto %d starting from %s' - show root instead of list repr

The debug statement is already in a 'for root in roots' so it was probably the
intention to show root instead of roots. Do that.

hgext/rebase.py

@@ -790,7 +790,7 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, roots))
+        repo.ui.debug('rebase onto %d starting from %s\n' % (dest, root))
         state.update(dict.fromkeys(rebaseset, nullrev))
         # Rebase tries to turn <dest> into a parent of <root> while
         # preserving the number of parents of rebased changesets:

tests/test-rebase-conflicts.t

@@ -210,7 +210,7 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 [<changectx e31216eec445>]
+  rebase onto 2 starting from e31216eec445
   rebasing: 9:e31216eec445 5/6 changesets (83.33%)
    future parents are 2 and -1
   rebase status stored