diff: avoid going from contexts to nodes and back This will allow us to pass in-memory contexts that may not have a valid node to the diffing logic. Differential Revision: https://phab.mercurial-scm.org/D8503

cleanup: avoid extra node/ctx conversions in logcmdutil.diffordiffstat I'm about to write some code that wants to pass a memctx to diffordiffstat, but this feels like a meritorious cleanup anyway, since the first thing this method does is turn nodes into contexts, and most callers have a context handy. Differential Revision: https://phab.mercurial-scm.org/D8502