rebase: don't use util.Abort for an internal error

hgext/rebase.py

@@ -90,7 +90,7 b' def rebase(ui, repo, **opts):'
         contf = opts.get('continue')
         abortf = opts.get('abort')
         collapsef = opts.get('collapse', False)
-        extrafn = opts.get('extrafn')
+        extrafn = opts.get('extrafn') # internal, used by e.g. hgsubversion
         keepf = opts.get('keep', False)
         keepbranchesf = opts.get('keepbranches', False)
         detachf = opts.get('detach', False)
@@ -138,8 +138,7 b' def rebase(ui, repo, **opts):'
                     external = checkexternal(repo, state, targetancestors)
 
         if keepbranchesf:
-            if extrafn:
-                raise util.Abort(_('cannot use both keepbranches and extrafn'))
+            assert not extrafn, 'cannot use both keepbranches and extrafn'
             def extrafn(ctx, extra):
                 extra['branch'] = ctx.branch()