convert a path to valid glob syntax when prefetching

Summary:
Paths are not necessarily legal glob syntax. In particular, backslash
is used for escaping. This caused problems on Windows, where we tried
to pass a backslash-delimited path into `eden prefetch --no-prefetch`.

Reviewed By: xavierd

Differential Revision: D25072784

fbshipit-source-id: 9ce8e5ccc8f28581512c39d04922889da0bc1bf6

Author: chadaustin

build/fbcode_builder/getdeps/copytree.py

@@ -59,11 +59,9 @@ def prefetch_dir_if_eden(dirpath):
     root = find_eden_root(dirpath)
     if root is None:
         return
-    rel = os.path.relpath(dirpath, root)
-    print("Prefetching %s..." % rel)
-    subprocess.call(
-        ["edenfsctl", "prefetch", "--repo", root, "--silent", "%s/**" % rel]
-    )
+    glob = f"{os.path.relpath(dirpath, root).replace(os.sep, '/')}/**"
+    print(f"Prefetching {glob}")
+    subprocess.call(["edenfsctl", "prefetch", "--repo", root, "--silent", glob])
     PREFETCHED_DIRS.add(dirpath)