t: add unreferenced objects to prune stash tests
t: add unreferenced objects to prune stash tests

We add unreferenced LFS objects on a separate branch
to our tests of the "git lfs prune" command when it
follows a "git stash" command, and expect that in all
cases the unreferenced objects will be pruned.

github/git-lfs,github/git-lfs,github/git-lfs