Fix markup heading parsing, fix emphasis parsing (#36284)

Fixes #36106, fix #17958

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/markdown/markdown_test.go

@@ -88,6 +88,7 @@ func TestRender_Images(t *testing.T) {
 }
 
 func TestTotal_RenderString(t *testing.T) {
+	setting.AppURL = AppURL
 	defer test.MockVariableValue(&markup.RenderBehaviorForTesting.DisableAdditionalAttributes, true)()
 
 	// Test cases without ambiguous links (It is not right to copy a whole file here, instead it should clearly test what is being tested)
@@ -258,7 +259,7 @@ This PR has been generated by [Renovate Bot](https://github.com/renovatebot/reno
 		},
 	})
 	for i := range sameCases {
-		line, err := markdown.RenderString(markup.NewTestRenderContext(localMetas), sameCases[i])
+		line, err := markdown.RenderString(markup.NewTestRenderContext(localMetas).WithEnableHeadingIDGeneration(true), sameCases[i])
 		assert.NoError(t, err)
 		assert.Equal(t, testAnswers[i], string(line))
 	}
@@ -545,5 +546,11 @@ func TestMarkdownLink(t *testing.T) {
 	assert.Equal(t, `<p><a href="/base/foo" rel="nofollow">link1</a>
 <a href="/base/foo" rel="nofollow">link2</a>
 <a href="#user-content-foo" rel="nofollow">link3</a></p>
+`, string(result))
+
+	input = "https://example.com/__init__.py"
+	result, err = markdown.RenderString(markup.NewTestRenderContext("/base", localMetas), input)
+	assert.NoError(t, err)
+	assert.Equal(t, `<p><a href="https://example.com/__init__.py" rel="nofollow">https://example.com/__init__.py</a></p>
 `, string(result))
 }