Set path to null for archive link of root tree

Prior to this change, the archive link for the root tree was of the
form "revision/.ext"; this makes it "revision.ext", meaning the
archive of the root tree is identical to (the same URL and therefore
contents as) the archive of the commit.

Change-Id: Ied963a6ecb8c5c8b054c18b1f76e59d980c24938