Windows build again

[Simon Peyton Jones]

Windows build is broken in a new way.
When I run validate I end up with sh.exe processes that consume a full CPU forever.   See the process log below.

Note that these are not GHC processes: they are shells!  I have no conception of what they are doing.
Any ideas, or things I can to do gather more evidence?
This is an up to date HEAD, with some small changes to GHC.  I suppose I can try a completely clean HEAD, but I can't see how my changes could make the shell loop.
Simon

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