Tests fail to run the second time through on Windows.

Regression tests fail the second time through because Git for Windows removes the files pointed to by links when "git clean -fdx" is run. See https://github.com/git-for-windows/git/issues/607.

To reproduce:
git clone --recursive https://github.com/boostorg/boost.git
cd boost
bootstrap.bat
b2 headers
At this point there is a subdirectory boost with links to library headers.

git clean -fdx
The links are deleted along with the header files they point to.

git submodule foreach git reset --hard
Puts the files back.