gitattributes: Suppress transformations in ud

We want all of our own manipulations to be unaffected by gitattributes
transformations, so that the imported git *trees* are identical to the
source packages.  (Ie, if transformations are in effect, the
*checkout* of such a tree might not be identical to the source
package.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>