code_excerpt_updater doesn't handle nested Markdown code blocks

[parlough]

If a Markdown code block (backticks) has an embedded markdown code block, the excerpts are sometimes repetitively added.

[parlough]

One example of this is

site-www/src/effective-dart/documentation.md

Lines 349 to 357 in 27c5221

	<?code-excerpt "docs_good.dart (code-sample)"?>
	{% prettify dart tag=pre+code %}
	/// Returns the lesser of two numbers.
	///
	/// ```dart
	/// min(5, 3) == 3
	/// ```
	num min(num a, num b) => ...
	{% endprettify %}

which has to still use a prettify block due to this.