Fix broken mercurial http post args configuration

[sdorra]

Proposed changes

Fixes a regression which was introduced with #1416. In #1416 we have reimplemented the way configuration is passed to the mercurial cgi handler. Before #1416 we used environment variables which are picked up by hgweb.py, after #1416 we pass mercurial configurations as command line parameters directly in the HgCGIServlet. But sadly the configuration option for httppostargs uses still an environment variable, which is not picked up by anyone.

See #1525

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