Simplify pack reading.

dulwich/__init__.py

@@ -17,5 +17,6 @@
 # MA  02110-1301, USA.
 
 import client
+import protocol
 import repo
 import server

dulwich/pack.py

@@ -364,8 +364,9 @@ class PackData(object):
     self._filename = filename
     assert os.path.exists(filename), "%s is not a packfile" % filename
     self._size = os.path.getsize(filename)
-    assert self._size >= 12, "%s is too small for a packfile" % filename
-    self._header_size = self._read_header()
+    self._header_size = 12
+    assert self._size >= self._header_size, "%s is too small for a packfile" % filename
+    self._read_header()
 
   def _read_header(self):
     f = open(self._filename, 'rb')
@@ -379,7 +380,6 @@ class PackData(object):
     (version,) = struct.unpack_from(">L", header, 4)
     assert version in (2, 3), "Version was %d" % version
     (self._num_objects,) = struct.unpack_from(">L", header, 8)
-    return 12 # Header size
 
   def __len__(self):
       """Returns the number of objects in this pack."""