Avoid 00changelog.i corruption

[Ingo Proetel]

Matt Mackall wrote:
> 
> There are lots of places we append to files, is this the only one
> affected? Are the nuls at the end of the file or somewhere else?
> 
Yes, there are several places where this could happen. So probably a more general solution is required:
A new class 'appender' util.py could be used to create a copy of the original file and return a special sub class of
posixfile. This subclass would forward the written data to the copy and on close() do the rename.

In most of the other places opener(xxx , 'a') would be replaced with appender(xxx).

Cheers,
Ingo

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