It's not so inconceivable, after seeing the PDF collision, to contribute to another project a commit whose hash has a collision with another malicious commit you keep up your sleeve. Not saying it's easy, but now it's on the horizon.

> Not saying it's easy, but now it's on the horizon.

Not really. It's not a preimage attack. They spent several hundred dollars to find two random byte strings with the same SHA1 hash. There's still no way to SHA1-collide a specific byte string instead of random junk.

This is exactly what euyyn is saying: create two files with the same SHA1 (by adding bytes of gibberish to an unused section), commit one to the repository, and now you have an collision available.

That's not how git uses hashes. In that scenario, there would still be a diff and hence git would recognize the files were different.