test: enforce non-general delta in 'test-generaldelta.t'

If general delta becomes the default, we need to be explicit about what we
want to be tested.

tests/test-generaldelta.t

@@ -62,7 +62,7 b' commit.'
   o  0 3903 a
   
   $ cd ..
-  $ hg init client
+  $ hg init client --config format.generaldelta=false
   $ cd client
   $ hg pull -q ../server -r 4
   $ hg debugindex x