manifest: remove optional default= argument on flags(path)

It had only one caller inside manifest.py, and treemanifest was
actually incorrectly implemented. treemanifest is still missing the
fastdelta() method from the interface (and so doesn't yet conform),
but this is at least progress.

Differential Revision: https://phab.mercurial-scm.org/D8069

mercurial/interfaces/repository.py

@@ -1027,8 +1027,8 b' class imanifestdict(interfaceutil.Interf'
     def get(path, default=None):
         """Obtain the node value for a path or a default value if missing."""
 
-    def flags(path, default=b''):
-        """Return the flags value for a path or a default value if missing."""
+    def flags(path):
+        """Return the flags value for a path (default: empty bytestring)."""
 
     def copy():
         """Return a copy of this manifest."""

mercurial/manifest.py

@@ -460,7 +460,7 b' class manifestdict(object):'
     __bool__ = __nonzero__
 
     def __setitem__(self, key, node):
-        self._lm[key] = node, self.flags(key, b'')
+        self._lm[key] = node, self.flags(key)
 
     def __contains__(self, key):
         if key is None:
@@ -595,11 +595,11 b' class manifestdict(object):'
         except KeyError:
             return default
 
-    def flags(self, key, default=b''):
+    def flags(self, key):
         try:
             return self._lm[key][1]
         except KeyError:
-            return default
+            return b''
 
     def copy(self):
         c = manifestdict()