rebase: don't update state dict same way for each root

The update statement does not depend on anything in the loop, so just
move it before the loop and do it once. There are no cases where
update would happen 0 times before (and 1 now); the function returns
early in all such cases.

hgext/rebase.py

@@ -1246,7 +1246,7 b' def buildstate(repo, dest, rebaseset, co'
     if not roots:
         raise error.Abort(_('no matching revisions'))
     roots.sort()
-    state = {}
+    state = dict.fromkeys(rebaseset, revtodo)
     detachset = set()
     for root in roots:
         commonbase = root.ancestor(dest)
@@ -1264,7 +1264,6 b' def buildstate(repo, dest, rebaseset, co'
                 return None
 
         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:
         #