[rakoo]

Yes it is, but it makes much more sense for git to use it: if a single byte is changed somewhere, you only need re-hashing the file and the trees up to the root; the other blobs/trees are left unchanged. It is very useful in this setting to reuse older existing hashes. In bittorrent, OTOH, we suppose every swarm will share different content, so there was no point making sure a hash would be reused somewhere.