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
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])