test-fileset: remove bashism, use test instead of '[[ ]]'

Debian dash complains about it.

  $TESTTMP.sh: 213: $TESTTMP.sh: [[: not found

tests/test-fileset.t

@@ -226,7 +226,7 b' saverev property is enough to alter the '
   b
   $ oldnode=`hg log -r tip -T "{node}\n"`
   $ newnode=`hg log -R ../converted -r tip -T "{node}\n"`
-  $ [[ "$oldnode" != "$newnode" ]] || echo "nothing changed"
+  $ [ "$oldnode" != "$newnode" ] || echo "nothing changed"
 
 Test with a revision