getdeps: use `eden prefetch` on windows

Summary:
Now that we've deployed the new eden build with globfiles
support, we can enable the use of eden prefetch in the getdeps
build when running inside an EdenFS mount on Windows.

Reviewed By: fanzeyi

Differential Revision: D21692689

fbshipit-source-id: b42e778901976cf0385ec31056c227b2049162dc

build/fbcode_builder/getdeps/copytree.py

@@ -54,10 +54,6 @@ def prefetch_dir_if_eden(dirpath):
     so we help accelerate things by performing a prefetch on the
     source directory """
     global PREFETCHED_DIRS
-    if is_windows():
-        # prefetch takes longer than not prefetching our opensource
-        # projects until we cut over to the new globfiles implementation
-        return
     if dirpath in PREFETCHED_DIRS:
         return
     root = find_eden_root(dirpath)