This is much of the plumbing for the object database to support SHA256,
and for objects to be able to parse SHA256 versions of themselves.

Teach the repository about the `objectformat` extension, supporting
`sha1` always and `sha256` when the experimental sha256 support is
active.

Teach the packfile machinery to cope with SHA256.

In clar, an underscore is meaningful; avoid using it incorrectly /
unnecessarily.

This is a conversion of the testrepo.git to SHA256 support.