Add list of commits to PR body for git submodules

[jordemort]

When Dependabot updates a Ruby gem, it gives me a changelog and a list of upstream commits in the PR body.

It would be great if it did something similar for git submodules, i.e. a list of commits in the submodule since it was last updated.

[jordemort]

Hm, this seems to have happened like I wanted it to in today's round of PRs.

Is the difference between submodules hosted on GitHub vs submodules that aren't?