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

mercurial/commands.py

@@ -2492,7 +2492,11 b' def diff(ui, repo, *pats, **opts):'
     node1, node2 = ctx1.node(), ctx2.node()
 
     if reverse:
-        node1, node2 = node2, node1
+        ctxleft = ctx2
+        ctxright = ctx1
+    else:
+        ctxleft = ctx1
+        ctxright = ctx2
 
     diffopts = patch.diffallopts(ui, opts)
     m = scmutil.match(ctx2, pats, opts)
@@ -2502,8 +2506,8 b' def diff(ui, repo, *pats, **opts):'
         ui,
         repo,
         diffopts,
-        repo[node1],
-        repo[node2],
+        ctxleft,
+        ctxright,
         m,
         stat=stat,
         listsubrepos=opts.get(b'subrepos'),