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.

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.