Two minor optimizations in heavily used code paths.

dulwich/pack.py

@@ -1182,8 +1182,10 @@ class PackData(object):
         and then the packfile can be asked directly for that object using this
         function.
         """
-        if offset in self._offset_cache:
+        try:
             return self._offset_cache[offset]
+        except KeyError:
+            pass
         assert isinstance(offset, long) or isinstance(offset, int),\
                 'offset was %r' % offset
         assert offset >= self._header_size

dulwich/walk.py

@@ -143,7 +143,7 @@ class _CommitTimeQueue(object):
             self._pq_set.remove(sha)
             if sha in self._done:
                 continue
-            self._done.add(commit.id)
+            self._done.add(sha)
 
             for parent_id in self._get_parents(commit):
                 self._push(parent_id)