Two ways to accomplish what you ask, depending what you want to get at:

git clone -b max https://git.osmocom.org/op25

------- or -------

git clone https://github.com/boatbod/op25


The first command gets you to the max branch of the official osmocom repo.  The second gets you to my fork of the osmocom repo.  In general my fork contains all of the max branch features (there may be a small lag of a few days to a couple weeks) plus a bunch of my own additions.  Pick the repo that best suits your purpose.