The "prune keep stashed changes", "prune keep stashed changes
in index", and "prune keep stashed untracked files" tests ensure
that Git LFS objects referenced by stashes are always retained
when the "git lfs prune" command is run.

However, they do not at present check that objects belonging to
files which match a "lfs.fetchexclude" pattern, which would
otherwise be pruned regardless of their status, will be retained
because they have been stashed.  Logic in the "git lfs prune"
command to retain objects from stashed changes was added in
PR git-lfs#4209, and it should demonstrate this behaviour, but no checks
actually confirm it.

We therefore expand these three tests so that their simulated
commit histories include objects whose file path matches a path we
then exclude using the "lfs.fetchexclude" filter.  We then confirm
that all these objects are retained by "git lfs prune" when they are
stashed, included when they are stashed from the index and when they
are untracked files stashed with "git stash -u".

The "prune worktree" test ensures that Git LFS objects
referenced by worktrees are always retained when the "git lfs
prune" command is run.

However, it does not at present check that objects belonging to
files which match a "lfs.fetchexclude" pattern are pruned even
when they are referenced by a commit at the HEAD of a worktree.
Logic in the "git lfs prune" command to prune objects from files
matching the "lfs.fetchexclude" filter was added in PR git-lfs#2851,
and it should demonstrate this behaviour, but no checks actually
confirm it.

We therefore expand the "prune worktree" test so that its
simulated commit history includes objects in each worktree's
branch whose file path matches a path we then exclude using the
"lfs.fetchexclude" filter.  We then confirm that these objects
are pruned by "git lfs prune", despite being referenced by the
worktrees' HEAD refs.