tpl: Fix it so we always prefer internal codeblock rendering over render-codeblock-foo.html and similar

Fixes #13651

tpl/tplimpl/templatedescriptor.go

@@ -130,17 +130,12 @@ func (this TemplateDescriptor) doCompare(category Category, isEmbedded bool, def
 
 	// One example of variant1 and 2 is for render codeblocks:
 	// variant1=codeblock, variant2=go (language).
-	if other.Variant1 != "" && other.Variant1 != this.Variant1 {
+	if other.Variant1 != "" {
-		return w
+		if other.Variant1 != this.Variant1 {
-	}
-
-	if isEmbedded {
-		if other.Variant2 != "" && other.Variant2 != this.Variant2 {
 			return w
 		}
-	} else {
+
-		// If both are set and different, no match.
+		if other.Variant2 != "" && other.Variant2 != this.Variant2 {
-		if other.Variant2 != "" && this.Variant2 != "" && other.Variant2 != this.Variant2 {
 			return w
 		}
 	}

tpl/tplimpl/templatestore_integration_test.go

@@ -1038,6 +1038,35 @@ _markup/render-codeblock-goat.html
 	b.AssertFileContent("public/index.html", "_markup/render-codeblock.html_markup/render-codeblock-goat.html")
 }
 
+func TestLookupCodeblockIssue13651(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/all.html --
+{{ .Content }}|
+-- layouts/_markup/render-codeblock-foo.html --
+render-codeblock-foo.html
+-- content/_index.md --
+---
+---
+
+§§§
+printf "Hello, world!"
+§§§
+
+§§§ foo
+printf "Hello, world again!"
+§§§
+`
+
+	b := hugolib.Test(t, files)
+
+	content := b.FileContent("public/index.html")
+	fooCount := strings.Count(content, "render-codeblock-foo.html")
+	b.Assert(fooCount, qt.Equals, 1)
+}
+
 // Issue #13515
 func TestPrintPathWarningOnDotRemoval(t *testing.T) {
 	t.Parallel()