This fixes an issue on Windows that causes build to fails when
fix_symlinks() is called concurrently with another build step.
It is also no longer necessary, since recent versions of git know how to
properly create symbolic links on checkout.
* Make sync_third_party work in general
* Un-break build.py and run_hooks.py on windows
* Partially fix format.py on windows
* Reduce code duplication between run_hooks and sync_third_party