Now that we have file attachment comments showing up as part of the diff, we also want to include that data in the diff comments hint. This change adds that in, expanding the file attachment comment helper function to return some additional data useful for this. This simplifies the client side a bit by allowing the tip revision to be null, for the case where we don't want the URL to include `?tip-commit-id`. Testing Done: Created a draft review with a bunch of comments on files in various diff revisions, interdiff revisions, and commit ranges (as well as some comments on regular file attachments). Saw that the diff comment hint correctly contained all the revisions and that clicking on everything loaded the correct page.