Clean up asserts in Tree.

Document the types of arguments to various tree methods instead of
asserting. In particular, these asserts were causing failures in
python2.4, when reading modes from the index resulted in a long rather
than an int. This is harmless, so the asserts were just dropped.

Change-Id: I4a8f8b1527e0fd9571ebdb760e09230f25ac3918

dulwich/objects.py

@@ -740,10 +740,16 @@ class Tree(ShaFile):
         return self._entries[name]
 
     def __setitem__(self, name, value):
-        assert isinstance(value, tuple)
-        assert len(value) == 2
+        """Set a tree entry by name.
+
+        :param name: The name of the entry, as a string.
+        :param value: A tuple of (mode, hexsha), where mode is the mode of the
+            entry as an integral type and hexsha is the hex SHA of the entry as
+            a string.
+        """
+        mode, hexsha = value
         self._ensure_parsed()
-        self._entries[name] = value
+        self._entries[name] = (mode, hexsha)
         self._needs_serialization = True
 
     def __delitem__(self, name):
@@ -760,9 +766,13 @@ class Tree(ShaFile):
         return iter(self._entries)
 
     def add(self, mode, name, hexsha):
-        assert type(mode) == int
-        assert type(name) == str
-        assert type(hexsha) == str
+        """Add an entry to the tree.
+
+        :param mode: The mode of the entry as an integral type. Not all possible
+            modes are supported by git; see check() for details.
+        :param name: The name of the entry, as a string.
+        :param hexsha: The hex SHA of the entry as a string.
+        """
         self._ensure_parsed()
         self._entries[name] = mode, hexsha
         self._needs_serialization = True