The git_object is what will be actually useful to the vault.
It's also easier to test, because test_identifier.py has the
entire git_object in its test data.
However, it makes the code messier, and it's quite a big change,
so I'm not sure it's worth it.
Instead, I could slightly rewrite the tests, and make the vault
re-add the header itself (but it would duplicate this code...)
Depends on D5650