Stale content issue on server rebuilds with Markdown content included with short refs in site.GetPage

[bep]

I had a head scratching moment investigating a bug in the wild.

Here's a failing test:

func TestRenderShortcodesIncludeShortRefEdit(t *testing.T) {
	t.Parallel()

	files := `
-- hugo.toml --
disableLiveReload = true
disableKinds = ["home", "taxonomy", "term", "section", "rss", "sitemap", "robotsTXT", "404"]
-- content/first/p1.md --
---
title: "p1"
---
## p1-h1
{{% include "p2" %}}
-- content/second/p2.md --
---
title: "p2"
---
### p2-h1
-- layouts/shortcodes/include.html --
{{ $p := site.GetPage (.Get 0) -}}
{{ $p.RenderShortcodes -}}
-- layouts/_default/single.html --
{{ .Content }}
`
	b := TestRunning(t, files)
	b.AssertFileContentEquals("public/first/p1/index.html", "<h2 id=\"p1-h1\">p1-h1</h2>\n<p></p>\n<h3 id=\"p2-h1\">p2-h1</h3>\n<p></p>\n")

	b.EditFileReplaceAll("content/second/p2.md", "p2-h1", "p2-h1-edited").Build()
	b.AssertFileContentEquals("public/first/p1/index.html", "<h2 id=\"p1-h1\">p1-h1</h2>\n<p></p>\n<h3 id=\"p2-h1-edited\">p2-h1-edited</h3>\n<p></p>\n")
}

The above test passes if you use the full path to the content file to include:

{{% include "second/p2" %}}