Fix directory_to_metadata on Windows.
Fix directory_to_metadata on Windows.

Windows doesn't like './' path and doesn't know how to follow symlinks.

R=maruel@chromium.org
BUG=

Review URL: https://codereview.appspot.com/100250043

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py