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>

modules/markup/renderer.go

@@ -82,9 +82,17 @@ type RenderContext struct {
 }
 
 type Links struct {
-	Base       string
-	BranchPath string
-	TreePath   string
+	AbsolutePrefix bool
+	Base           string
+	BranchPath     string
+	TreePath       string
+}
+
+func (l *Links) Prefix() string {
+	if l.AbsolutePrefix {
+		return setting.AppURL
+	}
+	return setting.AppSubURL
 }
 
 func (l *Links) HasBranchInfo() bool {