DARCS_PATCHES_XML does not appear to be set for Windows posthooks, why?
http://irclog.perlgeek.de/darcs/2010-04-06#i_2200266
One hypothesis is that it actually is being set, but that the sh-based
test does not really capture this. We need somebody on a Windows box to
try out a version of the test with a proper Windows-specific hook that
uses the Windows environment.