PlaceholdersPostFilter only updates <a href> when matches expected

What does this MR do and why?

<a href> is a security-relevant attribute; don't "update" it to match another value if it doesn't match the original value in the first place.

There is no outstanding security issue this addresses; it's a correctness and defense-in-depth matter that we should be doing anyway. Closes Only update image parent <a> href in if it matc... (#588831).

How to set up and validate locally

1. Check out the branch.
2. Enable the markdown_placeholders FF.
3. Enter the following Markdown into a comment/issue/MR description/etc.:

   [![](http://%{gitlab_server}:3000/apple-touch-icon.png)](https://elsewhere.example)

4. The image should display correctly, but it should still link to https://elsewhere.example, and not the image.

MR acceptance checklist

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