`fn sha1_zero` can be `const fn`, and since it does not take arguments,
it can just be a `const`. There is no good reason to not have a constant.
Justification for changing the order to first specify "zero" and then
"SHA1": In the future we will support multiple object formats. A
function that produces a zero OID would be parameterized by the object
format then. Imagine `fn zero(format: ObjectFormat)` which would be
called as `Oid::zero(ObjectForma::Sha1)`. In anticipation, we change the
ordering to match.
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