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 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.