Remove `lang` attribute from highlighted lines
What does this MR do and why?
This attribute is being used incorrectly and is not doing as it was intended to. Instead the files language has been moved to the diff file element instead. The copy as GFM code has been changed to query this diff file element to get the language of the file before copying to clipboard.
For diffs we also remove the un-needed <span class="line" />
element - for the
way we render diffs it isn't needed and bloats the payload.
For larger commits/merge requests the response should be quicker, I have seen
improvements of 5-10% reduction in payload size locally depending on the merge
request.
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