Use relative links for commits, mentions, and issues in markdown (#29427)

Fixes #29404

Use relative links for
- commits
- mentions
- issues

---------

Co-authored-by: silverwind <me@silverwind.io>

services/mailer/mail.go

@@ -222,7 +222,8 @@ func composeIssueCommentMessages(ctx *mailCommentContext, lang string, recipient
 	body, err := markdown.RenderString(&markup.RenderContext{
 		Ctx: ctx,
 		Links: markup.Links{
-			Base: ctx.Issue.Repo.HTMLURL(),
+			AbsolutePrefix: true,
+			Base:           ctx.Issue.Repo.HTMLURL(),
 		},
 		Metas: ctx.Issue.Repo.ComposeMetas(ctx),
 	}, ctx.Content)