Two ways to accomplish what you ask, depending what you want to get at:
git clone -b max
https://git.osmocom.org/op25 https://git.osmocom.org/op25
------- or -------
git clone
https://github.com/boatbod/op25 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.