test_loader: Fix implementation of test_loader_with_ref_delta_in_pack

Previous implementation was building an invalid pack file with REF_DELTA
object types as it was using the new object to deltify as the base of the
delta.

This was leading to errors and undefined behavior after building an index
for such a pack file as the deltified objects could not be properly resolved
by dulwich (observed by stsp while working on git loader improvements).

The bases for deltified objects are now objects that were previously loaded
into the archive.

Tag objects produced in that test are also ensured to be valid.

swh/loader/git/tests/test_loader.py

-# Copyright (C) 2018-2023  The Software Heritage developers
+# Copyright (C) 2018-2024  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -13,12 +13,13 @@ import subprocess
 import sys
 from tempfile import SpooledTemporaryFile
 from threading import Thread
+import time
 from unittest.mock import MagicMock, call
 
 import attr
 from dulwich.errors import GitProtocolError, NotGitRepository, ObjectFormatException
 from dulwich.pack import REF_DELTA
-from dulwich.porcelain import push
+from dulwich.porcelain import get_user_timezones, push
 import dulwich.repo
 from dulwich.tests.utils import build_pack
 import pytest
@@ -293,8 +294,12 @@ class TestGitLoader(FullGitLoaderTests, CommonGitLoaderNotFound):
         def add_tag(tag_name, tag_message, commit):
             tag = dulwich.objects.Tag()
             tag.name = tag_name
+            tag.tagger = b"John Doe <john.doe@example.org>"
             tag.message = tag_message
             tag.object = (dulwich.objects.Commit, commit)
+            tag.tag_time = int(time.time())
+            tag.tag_timezone = get_user_timezones()[0]
+            tag.check()
             self.repo.object_store.add_object(tag)
             self.repo[b"refs/tags/" + tag_name] = tag.id
             return tag
@@ -350,18 +355,30 @@ class TestGitLoader(FullGitLoaderTests, CommonGitLoaderNotFound):
         # get all object ids that will be in storage after third load
         objects_third_load = set(iter(self.repo.object_store))
 
-        # create a pack file containing full objects for newly added blob, tree,
-        # commit and tag in latest commit but also external references to objects
-        # that were discovered during the second loading of the repository
+        # create a pack file containing deltified objects for newly added blob, tree,
+        # commit and tag in latest commit whose bases are external objects that were
+        # discovered during the second loading of the repository
         objects = []
-        new_objects_second_load = objects_second_load - objects_first_load
-        new_objects_third_load = objects_third_load - objects_second_load
-        for obj_id in new_objects_third_load:
-            obj = self.repo.object_store[obj_id]
-            objects.append((obj.type_num, obj.as_raw_string()))
-        for obj_id in new_objects_second_load:
-            obj = self.repo.object_store[obj_id]
-            objects.append((REF_DELTA, (obj_id, obj.as_raw_string())))
+        new_objects_second_load = [
+            self.repo.object_store[obj_id]
+            for obj_id in (objects_second_load - objects_first_load)
+        ]
+        new_objects_third_load = [
+            self.repo.object_store[obj_id]
+            for obj_id in (objects_third_load - objects_second_load)
+        ]
+        for new_obj in new_objects_third_load:
+            base_obj = next(
+                obj
+                for obj in new_objects_second_load
+                if obj.type_num == new_obj.type_num
+            )
+            objects.append(
+                (
+                    REF_DELTA,
+                    (base_obj.id, new_obj.as_raw_string()),
+                )
+            )
         buffer = io.BytesIO()
         build_pack(buffer, objects, self.repo.object_store)