Links to commit sha in comment markdown is broken when you first make the comment

[bo-chen]

When a commit sha is included in a comment, its automatically converted to a link to the commit. Right now, the link is broken.
eg.
http://barkeep.sv2/commits//3003df144bbd30e630941fbc6232b3309392cf79

Looks like the repo part is missing, should we assume its always the same repo as the comment is made for?

[cespare]

OK, this is only broken when you initially make the comment. It's because it's rendered in a POST ("/comment") where the helper that does the replacement -- which looks at request.url -- doesn't have the url for the full page.

This isn't super high priority (because in general it will work when viewing the page).