git2graph: fix snapshot ID computation, now compatible with swh identify

Before this change the git header "snapshot LENGTH\0" was not added before
taking the SHA1, so results where (1) incompatible with swh identify, because of
(2) not comformant with the snapshot manifest specification.

With this change ...YAY!

Fix the tests accordingly, after cross-verifying test data with "swh identify
-t snapshot".