Ensure clone.bundle files have proper header

Server auth middleware may return a 200 from a clone.bundle request
that is not a bundle file, but instead a login or access denied page.
Instead of just checking the file size, actually check the first few
bytes of the file to ensure it is a bundle file before proceeding.

Change-Id: Icea07567c568a24fd838e5cf974c58f9e4abd7c0
1 file changed