Add support for long paths

* Add more file i/o wrappers in platform_utils to allow using
  long paths (length > MAX_PATH) on Windows.

* Paths using the long path syntax ("\\?\" prefix) should never
  escape the platform_utils API surface area, so that this
  specific syntax is not visible to the rest of the repo code base.

* Forward many calls from os.xxx to platform_utils.xxx in various place
  to ensure long paths support, specifically when repo decides to delete
  obsolete directories.

* There are more places that need to be converted to support long paths,
  this commit is an initial effort to unblock a few common use cases.

* Also, fix remove function to handle directory symlinks

Change-Id: If82ccc408e516e96ff7260be25f8fd2fe3f9571a

git_config.py

@@ -503,7 +503,7 @@ def close_ssh():
   d = ssh_sock(create=False)
   if d:
     try:
-      os.rmdir(os.path.dirname(d))
+      platform_utils.rmdir(os.path.dirname(d))
     except OSError:
       pass