Fix rsync -r on windows when src arg is the root of a volume.
Fix rsync -r on windows when src arg is the root of a volume.

Fix #295. Running `gsutil rsync -r D:\ gs://mybucket` would incorrectly
form file paths, missing the backslash between "D:" and the files under
that hierarchy, causing rsync to fail or incorrectly name the resulting
destination objects.

GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil