Replace occurences of darcs.haskell.org by git.haskell.org

...as the new `git.haskell.org` CNAME has been activated
(see also GitolitePlan and #8121 for more details)
Add note on isSafeOverlap field.
Update Win32 submodule.

This fixes a build failure for i386/Windows.

Signed-off-by: Austin Seipp <aseipp@pobox.com>