FIX Handle double underscored PR titles in changelog

[emteknetnz]

Issue https://github.com/silverstripeltd/product-issues/issues/844

There was a __CLASS__ in one of the PR titles. The new markdown linter didn't like this

[GuySartorelli]

I don't want to merge a partial solution like this. There are a bunch of other scenarios where the markdown will incorrectly format what should be unformatted content in commit messages.

We should either surround the whole commit in backticks (and we'd need to check if there are already backticks in the content and double-up the wrapper backticks if neccessary) or parse the markdown and backtick anything that gets rendered as non-plain-text.

[GuySartorelli]

There are also other options, such as having CI on PRs check commit messages and fail if the message has markdown that will render poorly.

Maybe open a card and we can discuss options.

[emteknetnz]

A better solution is desirable, though needs to be prioritized and there's no guarantee of that. This is still better than nothing and fixes an immediate problem. We can still merge this in the meantime and replace it later with a better solution.

Note: there's no guarantee that my laptop with this patch will be used to do the RC or the stable release, someone other than myself may do it, so I think we need something merge here.

[emteknetnz]

Have updated PR to test commit message for markdown, if it's markdown then surrond the whole message with backticks

[GuySartorelli]

Seems okay.