wireproto: use repo.lookup() for lookup command

I'm trying to reduce use of repo[<string>] and this seems like an
obvious place to use repo.lookup().

Differential Revision: https://phab.mercurial-scm.org/D3081

mercurial/wireproto.py

@@ -1014,8 +1014,8 b' def listkeys(repo, proto, namespace):'
 def lookup(repo, proto, key):
     try:
         k = encoding.tolocal(key)
-        c = repo[k]
-        r = c.hex()
+        n = repo.lookup(k)
+        r = hex(n)
         success = 1
     except Exception as inst:
         r = stringutil.forcebytestr(inst)