Add link to submodule diff when submodule refs change

Problem to solve

When looking at a diff (e.g. on an MR), changes to submodules are shown only as the change in the referenced commits:

mylibrary @ bbbbbbbb

- Subproject commit aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+ Subproject commit bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

While the heading is a link to the submodule project, it's hard to actually see what changed in the submodule.

Proposal

Simply add a link to submodule/compare?from=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa&to=bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb.

This will make it a one-click task to see what changed in the submodule between the points of reference in the super project.