repo: Add require_rev method

The `resolve_rev` C method should really have been
`resolve_rev_optional` from the start - it is more obviously wrong
in Rust because the input parameter `allows_noent` controls
whether the returned `Option` can ever be `None`.

I debated adding this to the C bindings, and may still do so,
but eh it's faster to write + ship in Rust, and the future of ostree is
Rust anyways.

rust-bindings/rust/tests/repo/mod.rs

@@ -10,9 +10,13 @@ mod checkout_at;
 fn should_commit_content_to_repo_and_list_refs_again() {
     let test_repo = TestRepo::new();
 
+    assert!(test_repo.repo.require_rev("nosuchrev").is_err());
+
     let mtree = create_mtree(&test_repo.repo);
     let checksum = commit(&test_repo.repo, &mtree, "test");
 
+    assert_eq!(test_repo.repo.require_rev("test").unwrap(), checksum);
+
     let repo = ostree::Repo::new_for_path(test_repo.dir.path());
     repo.open(NONE_CANCELLABLE).expect("OSTree test_repo");
     let refs = repo