bugdir: add `update` argument to BugDir.append().

bugdir: add `update` argument to BugDir.append().

This avoids a deepcopy error where the BugDir tries to update before
the Bug has had it's uuid assigned:

  Traceback (most recent call last):
    ...
    File ".../libbe/command/merge.py", line 168, in _run
      newCommTree = copy.deepcopy(bugB.comment_root)
    File "/usr/lib64/python2.7/copy.py", line 190, in deepcopy
      y = _reconstruct(x, rv, 1, memo)
    ...
    File "/usr/lib64/python2.7/copy.py", line 352, in _reconstruct
      y.append(item)
    File ".../libbe/bugdir.py", line 263, in append
      self._bug_map_gen()
    File ".../libbe/bugdir.py", line 152, in _bug_map_gen
      map[bug.uuid] = bug
  AttributeError: 'Bug' object has no attribute 'uuid'