localrepo: Support testing with SHA256
This MR creates the groundwork to allow us to test the localrepo package with the SHA256 object hash. There are two ommissions: CreateCommit()
depends on the catfile
package, and the remote-based tests depend on the CreateRepository()
RPC that doesn't yet support creating SHA256-based repos.
Closes #4354 (closed).
Edited by Patrick Steinhardt