patch: use ctx.node() instead of bare node variable

Future patches will allow patch.diff to take a basectx so node1 (and node2)
could make hexfunc error out. Instead, we'll call the node function on the
context object directly.

mercurial/patch.py

@@ -1622,7 +1622,7 b' def diff(repo, node1=None, node2=None, m'
 
     revs = None
     hexfunc = repo.ui.debugflag and hex or short
-    revs = [hexfunc(node) for node in [node1, node2] if node]
+    revs = [hexfunc(node) for node in [ctx1.node(), ctx2.node()] if node]
 
     copy = {}
     if opts.git or opts.upgrade: