Add Index to comment for migrations and mirroring (#18806)

Comments have an id (see Gitea[0], GitLab[1], GitHub[2], etc.), and the
comment migration format must represent it during migrations so that
it can be used during mirroring or incremental migrations.

[0] https://try.gitea.io/api/swagger#/issue/issueGetComment
[1] https://docs.gitlab.com/ee/api/discussions.html#get-single-issue-discussion-item
[2] https://docs.github.com/en/rest/reference/issues#get-an-issue-comment

Signed-off-by: Loïc Dachary <loic@dachary.org>
Co-authored-by: Loïc Dachary <loic@dachary.org>

integrations/dump_restore_test.go

@@ -178,7 +178,9 @@ func (c *compareDump) assertEquals(repoBefore, repoAfter *repo_model.Repository)
 	assert.GreaterOrEqual(c.t, len(issues), 1)
 	for _, issue := range issues {
 		filename := filepath.Join("comments", fmt.Sprintf("%d.yml", issue.Number))
-		comments, ok := c.assertEqual(filename, []base.Comment{}, compareFields{}).([]*base.Comment)
+		comments, ok := c.assertEqual(filename, []base.Comment{}, compareFields{
+			"Index": {ignore: true},
+		}).([]*base.Comment)
 		assert.True(c.t, ok)
 		for _, comment := range comments {
 			assert.EqualValues(c.t, issue.Number, comment.IssueIndex)