gbp-clone: set defuse-gitattributes to default to off

The use of gbp setup-gitattributes is known to break builds that would otherwise work perfectly fine on salsa (see 1), possibly with a greater frequency than the issues that motivated the implementation of that feature.

It can cause other issues like breaking test input data or script files intended for other operating systems.

It should really be reserved to the rare cases where the upstream configuration of .gitattributes interferes severely with the packaging work.

Gbp-Dch: Full Closes: #1092800