Markdown Video doesn't work when the referenced video file is in same repo

See https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#videos.

The <video> tag src is img/video.mp4, instead of https://gitlab.com/gitlab-org/gitlab-ce/raw/master/doc/markdown/img/video.mp4 (note the /raw/).

This does currently work for <img> tags, because of the RelativeLinkFilter filter.