apply tests: test delta callback errors

Test that we can return an error from the apply delta callback and the
error code is propagated back to the caller.

tests/apply/callbacks.c

+#include "clar_libgit2.h"
+#include "apply_helpers.h"
+
+static git_repository *repo;
+
+#define TEST_REPO_PATH "merge-recursive"
+
+void test_apply_callbacks__initialize(void)
+{
+	git_oid oid;
+	git_commit *commit;
+
+	repo = cl_git_sandbox_init(TEST_REPO_PATH);
+
+	git_oid_fromstr(&oid, "539bd011c4822c560c1d17cab095006b7a10f707");
+	cl_git_pass(git_commit_lookup(&commit, repo, &oid));
+	cl_git_pass(git_reset(repo, (git_object *)commit, GIT_RESET_HARD, NULL));
+	git_commit_free(commit);
+}
+
+void test_apply_callbacks__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+static int delta_abort_cb(const git_diff_delta *delta, void *payload)
+{
+	GIT_UNUSED(payload);
+
+	if (!strcmp(delta->old_file.path, "veal.txt"))
+		return -99;
+
+	return 0;
+}
+
+void test_apply_callbacks__delta_aborts(void)
+{
+	git_diff *diff;
+	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
+
+	opts.delta_cb = delta_abort_cb;
+
+	cl_git_pass(git_diff_from_buffer(&diff,
+		DIFF_MODIFY_TWO_FILES, strlen(DIFF_MODIFY_TWO_FILES)));
+	cl_git_fail_with(-99,
+		git_apply(repo, diff, GIT_APPLY_LOCATION_INDEX, &opts));
+
+	validate_index_unchanged(repo);
+	validate_workdir_unchanged(repo);
+
+	git_diff_free(diff);
+}