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I don't think this impacts libgit2 in any meaningful way. It only uses markdown-it for generating its documentation HTML. I can't see any obvious way for this to be exploited, since the input is well-validated (coming from the upstream repository).
markdown-it isn't even present in this package, nor as a dependency of it. It's just listed as a dependency in under script/api-docs/package.json, so this report is an utter false positive.