found an acceptable hack: include the commit id and file path as hints in the request uri. when the site generates a link to a git diff or git blob, it will include those hints, getting them from the surrounding context. if those hints are present when rendering the page, we can offer the option to create line-comments. then the expensive and difficult function doesn't have to be implemented and diffs and git blob pages can remain the low-level way with git blob ids (vs. commit ids + path).