merge: replace a repo.lookup('.') by more typical repo['.'].node()

The `repo.lookup('.')` form comes from b3311e26f94f (merge: fix
--preview to show all nodes that will be merged (issue2043).,
2010-02-15). I don't know why that commit changed from `repo['.']`,
but I don't think there's any reason to do that. Note that performance
should not be a reason (anymore?), because repo.lookup() is
implemented by first creating a context object.

Differential Revision: https://phab.mercurial-scm.org/D7998

mercurial/commands.py

@@ -4881,7 +4881,7 b' def merge(ui, repo, node=None, **opts):'
 
     if opts.get(b'preview'):
         # find nodes that are ancestors of p2 but not of p1
-        p1 = repo.lookup(b'.')
+        p1 = repo[b'.'].node()
         p2 = node
         nodes = repo.changelog.findmissing(common=[p1], heads=[p2])