Git uses hashes that include more data than just the hash of the content of an object. Meaning the existing known collisions don't work and you'd have to develop collisions specifically to target git.
Additionally, the current known collision attacks require generating the two colliding contents as a pair. You can't take an existing document and then generate a collision for that document. Further, the technique is not particularly good for text documents because you need to include some data to force the collision. This is easily missed in binary documents but not the text documents most people are committing into git repos (granted you can put binary files into git and that would still work but the other reasons for this not being very useful still apply).
Git also has code to deal with possible collisions and will fall back to full comparison and will fail a commit that creates a collision. Finally, generating these document pairs leaves detectable patterns in the content. GitHub scans for these patterns as part of their process.
https://github.blog/2017-03-20-sha-1-collision-detection-on-...