cmdutil: pass node instead of ctx to diffordiffstat

93bcc73df8d5 changed showpatch to use ctx's more, but it accidentally passed
prev as a context and node as a binary string, when both should be passed as
binary strings (since diffordiffstat tries to resolve them via repo[X]).

This affected hggit since the existing ctx belongs to the git overlay, but the
resolved context (from the repo[X] resolution) should belong to the main repo.
This broke a test because it tried to look in the git repo for data that didn't
exist.

This feels like a deeper issue in hggit somewhere, but the fix is here trivial and
obviously more correct

mercurial/cmdutil.py

@@ -1308,7 +1308,7 b' class changeset_printer(object):'
             diff = self.diffopts.get('patch')
             diffopts = patch.diffallopts(self.ui, self.diffopts)
             node = ctx.node()
-            prev = ctx.p1()
+            prev = ctx.p1().node()
             if stat:
                 diffordiffstat(self.ui, self.repo, diffopts, prev, node,
                                match=matchfn, stat=True)