Shallow (#6396) with some fixes from review

The `depth` field is suitable to specify unshallowing; provide an enum
to aide in specifying the `unshallow` value.

Users should provide us an array of object ids; we don't need a separate
type. And especially, we should not be mutating user-providing values.
Instead, use `git_oid *` in the shallow code.

Depth of `0` should indicate full depth. Disallow negative values (they
may have a future meaning) and use `0` as the default.

The opt mechanism isn't _really_ meant to be for feature flags, and it's
weird to feature flag shallow / unshallow at all.

sha256: less hardcoded SHA1 types and lengths