Setup read-only vendor SVN repository as mirror/external within my own repository

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http://stackoverflow.com – Currently I am working on opencart site and I would like to integrate opencarts source into my own SVN (https://github.com/opencart/opencart). The goal is to be able to make my own customizations to core OpenCart files and then, when a new version of open cart is released, integrate there changes into my local version. I was thinking of source structure as follows: /trunk/opencart - my custom version of open cart /vendor/opencart - the current opencart trunk. The intended workflow would be to pull the latest open cart from their servers, then perform a local merge (HowTos)