Support a single directory traversal in RelativeLinkFilter

Prior, if we were viewing a blob at https://example.com/namespace/project/blob/master/doc/some-file.md and it contained a relative link such as [README](../README.md), the resulting link when viewing the blob would be: https://example.com/namespace/project/blob/README.md which omits the master ref, resulting in a 404.

Fixes https://gitlab.com/gitlab-org/release-tools/issues/3