cat: fix cat without -r, broken by 0ae35296fbf4

The default revision for revsingle() is now '.' instead of None. This
preserves the behavior of cat prior to it using revsingle().

mercurial/cmdutil.py

@@ -111,7 +111,7 b' def loglimit(opts):'
         limit = None
     return limit
 
-def revsingle(repo, revspec, default=None):
+def revsingle(repo, revspec, default='.'):
     if not revspec:
         return repo[default]