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>

services/migrations/gogs.go

@@ -236,6 +236,7 @@ func (g *GogsDownloader) GetComments(opts base.GetCommentOptions) ([]*base.Comme
 		}
 		allComments = append(allComments, &base.Comment{
 			IssueIndex:  opts.Context.LocalID(),
+			Index:       comment.ID,
 			PosterID:    comment.Poster.ID,
 			PosterName:  comment.Poster.Login,
 			PosterEmail: comment.Poster.Email,