http://joeyh.name/blog/entry/size_of_the_git_sha1_collision_attack_surface/ http://joeyh.name/blog/entry/size_of_the_git_sha1_collision_... has some discussion

http://joeyh.name/blog/entry/size_of_the_git_sha1_collision_attack_surface/ http://joeyh.name/blog/entry/size_of_the_git_sha1_collision_... has some discussion of colliding tree objects.