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