Fix backtick excludes in CommitLinter
What does this MR do and why?
Fix the backtick excludes in CommitLinter so I can talk about GitLab references in commit messages. (I hit this a bit as Markdown DRI.)
Negative lookbehind assertions don't work if we don't actually match anything after, so we use lookahead assertions instead. We have to make the previous part of the match atomic to stop it just backtracking a character instead.
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