merge: avoid unnecessary conversion from binary nodeid to binary nodeid

"node" is already a binary nodeid here, so there's no need to convert
it. repo.lookup() will soon lose support for binary nodeids as input.

Differential Revision: https://phab.mercurial-scm.org/D3190

mercurial/commands.py

@@ -3587,7 +3587,7 def merge(ui, repo, node=None, **opts):
     if opts.get('preview'):
         # find nodes that are ancestors of p2 but not of p1
         p1 = repo.lookup('.')
-        p2 = repo.lookup(node)
+        p2 = node
         nodes = repo.changelog.findmissing(common=[p1], heads=[p2])
 
         displayer = logcmdutil.changesetdisplayer(ui, repo, opts)