[merge] Tweaks for AtomicFile and LocalTransport.append()

[John Arbash Meinel]

The attached patches tweak AtomicFile and LocalTransport.append and
mkdir so that hopefully they will ultimately do fewer chmod() calls. In
general, it streamlines a few code paths. The final performance
difference can be seen with 'make_kernel_like_tree'.

On my machine 'make_kernel_like_tree' takes:
  bzr.dev:		7.8 - 8.2
  with tweaks and put:	4.5 - 7.5
  switching to append:  2.9 - 4.5

I'm not really interested in the make_kernel_like_tree improvements,
since my benchmark caching will handle that even better. However, it
does show a general improvement in LocalTransport.put() and append()
speeds, which I am very interested in.

Robert started this work. I just cleaned it up, added direct tests for
AtomicFile, and made sure that file modes still worked correctly even
when umask is involved.

John
=:->
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