commit: correctly check commit mutability during commit --amend

The right way to check if a context is mutable is to call .mutable(),
not to compare .phase() with public.

mercurial/commands.py

@@ -1409,7 +1409,7 b' def commit(ui, repo, *pats, **opts):'
             raise util.Abort(_('cannot amend with ui.commitsubrepos enabled'))
 
         old = repo['.']
-        if old.phase() == phases.public:
+        if not old.mutable():
             raise util.Abort(_('cannot amend public changesets'))
         if len(repo[None].parents()) > 1:
             raise util.Abort(_('cannot amend while merging'))