Use batch insert on migrating repository to make the process faster (#7050)

* Use batch insert on migrating repository to make the process faster

* fix lint

* fix tests

* fix comments

modules/migrations/github.go

@@ -358,6 +358,7 @@ func (g *GithubDownloaderV3) GetComments(issueNumber int64) ([]*base.Comment, er
 				reactions = convertGithubReactions(comment.Reactions)
 			}
 			allComments = append(allComments, &base.Comment{
+				IssueIndex:  issueNumber,
 				PosterName:  *comment.User.Login,
 				PosterEmail: email,
 				Content:     *comment.Body,