prefetch projects in the background

Summary:
There's no reason to block a getdeps build on prefetching its sources,
so issue all prefetches in the background.

Reviewed By: genevievehelsel

Differential Revision: D33855396

fbshipit-source-id: 1ba01b0587e9bc0e74e6bba5b8571af76bf2516d

build/fbcode_builder/getdeps/copytree.py

@@ -59,7 +59,9 @@ def prefetch_dir_if_eden(dirpath):
         return
     glob = f"{os.path.relpath(dirpath, root).replace(os.sep, '/')}/**"
     print(f"Prefetching {glob}")
-    subprocess.call(["edenfsctl", "prefetch", "--repo", root, "--silent", glob])
+    subprocess.call(
+        ["edenfsctl", "prefetch", "--repo", root, "--silent", glob, "--background"]
+    )
     PREFETCHED_DIRS.add(dirpath)