manifestdict: drop empty-string argument when creating empty manifest

manifestdict() creates an empty manifestdict, so let's consistently
use that instead of explicitly parsing an empty string (which does
result in an empty manifest).

mercurial/manifest.py

@@ -270,7 +270,7 b' class manifestdict(object):'
                     m._lm[fn] = lm[fn]
             return m
 
-        m = manifestdict('')
+        m = manifestdict()
         m._lm = self._lm.filtercopy(match)
         return m
 
@@ -307,7 +307,7 b' class manifestdict(object):'
             return default
 
     def copy(self):
-        c = manifestdict('')
+        c = manifestdict()
         c._lm = self._lm.copy()
         return c