bzr pack and 2a format

[Alexander Belchenko]

I've noticed that `bzr pack` in 2a branch/repo no more is no-op 
operation when there is only one pack file present. I can run `bzr pack` 
many times in already packed branch and every time it starts again.

Also on subsequent repack of packed repo `bzr pack` blows with error:

C:\Temp\qbzr-bbc\trunk-2a>bzr pack


C:\Temp\qbzr-bbc\trunk-2a>bzr pack
bzr: ERROR: File exists: 
u'C:/Temp/qbzr-bbc/trunk-2a/.bzr/repository/upload/hz5plvk08aqz10duwmal.autopack': 
[Error 183] Cannot create a file when that file already exists

C:\Temp\qbzr-bbc\trunk-2a>bzr pack
bzr: ERROR: File exists: 
u'C:/Temp/qbzr-bbc/trunk-2a/.bzr/repository/upload/gmki2l0f66pj13kol7j7.autopack': 
[Error 183] Cannot create a file when that file already exists