files: replace 'ctx._repo' with 'ctx.repo()'

mercurial/cmdutil.py

@@ -2222,7 +2222,7 b' def forget(ui, repo, match, prefix, expl'
 def files(ui, ctx, m, fm, fmt):
     rev = ctx.rev()
     ret = 1
-    ds = ctx._repo.dirstate
+    ds = ctx.repo().dirstate
 
     for f in ctx.matches(m):
         if rev is None and ds[f] == 'r':