Merged revisions 71875 via svnmerge from

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  r71875 | walter.doerwald | 2009-04-25 14:15:07 +0200 (Sa, 25 Apr 2009) | 7 lines

  Issue #5837: Certain sequences of calls to set() and unset() for
  support.EnvironmentVarGuard objects restored the environment variables
  incorrectly on __exit__.

  Fix this by recording the initial value of each environment variable on the
  first access in set() or unset().
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