Make .RenderString render shortcodes

Fixes #6703

common/text/transform.go

@@ -64,11 +64,12 @@ func Puts(s string) string {
 
 // VisitLinesAfter calls the given function for each line, including newlines, in the given string.
 func VisitLinesAfter(s string, fn func(line string)) {
-	high := strings.Index(s, "\n")
+	high := strings.IndexRune(s, '\n')
 	for high != -1 {
 		fn(s[:high+1])
 		s = s[high+1:]
-		high = strings.Index(s, "\n")
+
+		high = strings.IndexRune(s, '\n')
 	}
 
 	if s != "" {

common/text/transform_test.go

@@ -59,3 +59,18 @@ line 3`
 	c.Assert(collected, qt.DeepEquals, []string{"line 1\n", "line 2\n", "\n", "line 3"})
 
 }
+
+func BenchmarkVisitLinesAfter(b *testing.B) {
+	const lines = `line 1
+	line 2
+	
+	line 3`
+
+	for i := 0; i < b.N; i++ {
+		VisitLinesAfter(lines, func(s string) {
+
+		})
+
+	}
+
+}