Add action that verifies Rust git dependency commits will not be orphaned

[leighmcculloch]

What

Add action that checks git rev deps for whether they are likely to be orphaned.

Why

On a number of occassions we've accidentally committed a reference to a dependency that belongs to a commit on a pull request. The pull request was then closed, and while the commit lives forever in the Git repository, it is not seen by tools that do not fetch orphaned commits.