Improve repository file reference filter in GFM
Description
It is possible to use a reference filter in a markdown like:
[README.md](README.md)It is going to link to a README on a branch like master.
When you write it like
[README.md](/README.md)it generates an invalid link like https://gitlab.com/gitlab-org/gitlab-qa/master/README.md which is not correct, because there is no blob path component.
It is quite useful feature, but there is no way to link to a relative GitLab page using this strategy ([pipeline](/gitlab-org/gitlab-ce/pipelines/1 doesn't work).
We already have a feature implemented that expected it to work, but it does not since GFM overrides a default behavior (create an issue when a build failed feature).