convert/mtn: avoid unnecessary initial test of mtn repo

As soon as before() is called, we'll know if there is a problem (and get a
better error message).

hgext/convert/monotone.py

@@ -20,6 +20,7 b' class monotone_source(converter_source, '
         self.ui = ui
         self.path = path
         self.automatestdio = False
+        self.rev = rev
 
         norepo = NoRepo(_("%s does not look like a monotone repository")
                         % path)
@@ -65,14 +66,6 b' class monotone_source(converter_source, '
 
         checktool('mtn', abort=False)
 
-        # test if there are any revisions
-        self.rev = None
-        try:
-            self.getheads()
-        except:
-            raise norepo
-        self.rev = rev
-
     def mtnrun(self, *args, **kwargs):
         if self.automatestdio:
             return self.mtnrunstdio(*args, **kwargs)