Fix modman

Prevent linking empty folder when Credis is already
linked (e.g. from Cm_Cache_Backend_Redis module)

After last change in modman file - linking Credis submodule, I've got some errors because I already had Credis module linked from Cache backend module and I havent checked it out again for Sessions module.
This patch makes it safe to checkout Credis only in one redis module (cache or sessions)