amend: use ctx1.status(ctx2) instead of repo.status(ctx1, ctx2)

Differential Revision: https://phab.mercurial-scm.org/D3996

mercurial/cmdutil.py

@@ -2428,7 +2428,7 b' def amend(ui, repo, old, extra, pats, op'
         if len(old.parents()) > 1:
             # ctx.files() isn't reliable for merges, so fall back to the
             # slower repo.status() method
-            files = set([fn for st in repo.status(base, old)[:3]
+            files = set([fn for st in base.status(old)[:3]
                          for fn in st])
         else:
             files = set(old.files())