repository: disentangle shallow and normal grafts

When loading shallow grafts, we add each of the grafting commits to the
grafts backed by ".git/info/grafts". Keeping track of both grafts
separately, but partially storing them in a common grafts structure is
likely to lead to inconsistencies. In fact, there already are
inconsistencies if refreshing shallow grafts at a later point, as we
only refresh the shallows, but not the normal grafts in that case.

Disentangle both grafting stores and instead check both separately when
parsing commits.

src/commit.c

@@ -504,7 +504,8 @@ int git_commit__parse_ext(git_commit *commit, git_odb_object *odb_obj, unsigned
 		return error;
 
 	/* Perform necessary grafts */
-	if (git_grafts_get(&graft, repo->grafts, git_odb_object_id(odb_obj)) != GIT_ENOTFOUND) {
+	if (git_grafts_get(&graft, repo->grafts, git_odb_object_id(odb_obj)) == 0 ||
+	    git_grafts_get(&graft, repo->shallow_grafts, git_odb_object_id(odb_obj)) == 0) {
 		size_t idx;
 		git_oid *oid;
 		git_array_clear(commit->parent_ids);

src/repository.c

@@ -613,22 +613,15 @@ cleanup:
 
 static int load_shallow(git_repository *repo)
 {
-	git_array_oid_t parents = GIT_ARRAY_INIT;
 	git_oidarray roots;
 	int error;
-	size_t i;
 
 	/* Graft shallow roots */
 	if ((error = git_repository_shallow_roots(&roots, repo)) < 0)
-		goto out;
-
-	for (i = 0; i < roots.count; i++)
-		if ((error = git_grafts_add(repo->grafts, &roots.ids[i], parents)) < 0)
-			goto out;
+		return error;
 
-out:
 	git_oidarray_free(&roots);
-	return error;
+	return 0;
 }
 
 int git_repository_open_bare(