filedata: use mercurial.util.posixfile

instead of Python's open().

Python's silly open() does not set FILE_SHARE_DELETE on the dwShareMode for the
CreateFile API function of Windows. Deleting files will fail in that state on
Windows.

As usual with this stuff, I cannot prove anything conclusive that this exact
open() call was truly bad here and the file is closed quickly anyway, but *not*
setting FILE_SHARE_DELETE when reading a file on Windows is a stupid thing.

One day, I might finally convince myself to try having a chat with the
Python folks about fixing their open(). Until that happens, I'm using
Mercurial's posixfile.

See also http://mercurial.selenic.com/wiki/UnlinkingFilesOnWindows