It depends on the level of certainty you are asking for in using the term 'know'.
No one has demonstrated the ability to find a collision with a pre-existing text in the open literature yet as far as I know. There are techniques to find generate a collision with less than 2^80 operations, but that's still a ways from attacking Git.
Even if someone had the ability to generate a collision with a preimage text, they'd have the secondary task of making the colliding text look reasonable to kernel developers. When people were having fun creating md5 collisions, they tended to have pretty long sequences of random bytes in the text, which would be hard to hide in a kernel source file.
Long story short, it's very unlikely that anyone out there has successfully attacked SHA-1 to the degree necessary to be able to attack the kernel's Git repo. If they had, it's unlikely that they'd have made the kind of mistakes that attracted kernel.org's attention.