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

--- a/mercurial/cmdutil.py	Sun Dec 27 23:37:14 2015 +0900
+++ b/mercurial/cmdutil.py	Mon Jan 04 16:14:04 2016 -0800
@@ -1308,7 +1308,7 @@
             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)