versioncontrol_svn_get_item() should store constraints

Since the this backend does not implement all of the possible versioncontrol API hooks, and some of those depend on versioncontrol_svn_get_item(), the $constraints argument should be stored in the returned array so that it can be used by other modules which may need this information for custom implementations of these missing API functions..

The textbook case is versioncontrol_svn_export_directory(), which would need the '$label' array that can be passed to specify a branch or tag to be exported.

The attached patch simply takes the $constraints array as it is received and stores it in the $item array.

A forthcoming issue will show how I used this to provide integration with versioncontrol_release module.