update-kwsys: Disable custom gitattributes during import

The KWSys `.gitattributes` file now defines custom attributes.
Git does not support these in a subdirectory where we are about
to place the content, so comment them out during import.
This also ensures that the `our-c-style` attribute on KWSys
sources does not activate the `format.clang-format` attribute
on the third-party import branch.

Utilities/Scripts/update-kwsys.bash

@@ -15,6 +15,7 @@ readonly paths="
 
 extract_source () {
     git_archive
+    disable_custom_gitattributes
 }
 
 export HOOKS_ALLOW_KWSYS=1