Use coverage before changeset when on tip

[La0]

A coverage request on the changeset at tip (or any changeset between the last push & tip) will give a 404.

Right now the coverage backend only looks for coverage data after the requested changeset. But that does not work in the case of tip, when no coverage data is yet available.

For that case (when changeset's push is above last changeset) the backend should fall back to the previous coverage data