rebase: restore i18n of a hint message

As noticed by Yuya, I lost the _() call in 71fee4564410 (rebase: use
rewriteutil.precheck() instead of reimplementing it, 2019-12-18).

Differential Revision: https://phab.mercurial-scm.org/D7697

hgext/rebase.py

@@ -399,7 +399,7 b' class rebaseruntime(object):'
                 rewriteutil.precheck(self.repo, rebaseset, action=b'rebase')
             except error.Abort as e:
                 if e.hint is None:
-                    e.hint = b'use --keep to keep original changesets'
+                    e.hint = _(b'use --keep to keep original changesets')
                 raise e
 
         result = buildstate(self.repo, destmap, self.collapsef)