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

See merge request !2247