RemoveReference should clean up empty directories

Start with the filesystem storage implementations of SetReference and RemoveReference, then use the provided TestConflictWithDeletedRef reproduction as the first test to run. Done means removing a nested reference also removes empty parent directories, allowing a later reference named bugfix to be created successfully.

Bug Description

When SetReference is called to create a reference with / in the name, it creates subdirectories for each part of the name separated by a slash. For example, when creating the ref bugfix/issue-1 SetReference would create it at the path <repo>/refs/bugfix/issue-1, creating the file issue-1 and the parent directory bugfix/.

When RemoveReference is called on the same reference, it deletes the reference but not the parent directories, even though they may be empty. In the example above, it would delete issue-1, but the refs/heads/bugfix/ directory would still exist.

Since the parent directories are not cleaned up, it becomes impossible to create any new references that share the same name. In the example above, it is now not possible to use SetReference to create a ref with the name bugfix, since it conflicts with the leftover directory refs/heads/bugfix/.

The Git implementation does cleanup parent directories when a branch is deleted.

go-git Version

v5.16.4

Steps to Reproduce

func TestConflictWithDeletedRef(t *testing.T) {
	repoPath := t.TempDir()
	fs := filesystem.NewStorage(osfs.New(repoPath), cache.NewObjectLRUDefault())
	if _, err := git.Init(fs, nil); err != nil {
		t.Fatalf("Failed to init git: %v", err)
	}

	// Create a new branch named bugfix/1.
	nestedRef := plumbing.NewReferenceFromStrings("refs/heads/bugfix/1", "target1")
	if err := fs.SetReference(nestedRef); err != nil {
		t.Fatalf("Failed to set reference: %v", err)
	}

	if err := fs.RemoveReference(nestedRef.Name()); err != nil {
		t.Fatalf("Failed to remove reference: %v", err)
	}

	// Now we try to create a ref named 'bugfix' but it fails, since the directory refs/heads/bugfix/ exists.
	// Error:
	// Failed to set reference: open /tmp/TestConflictWithDeletedRef4236784409/001/refs/heads/bugfix: is a directory
	conflictingRef := plumbing.NewReferenceFromStrings("refs/heads/bugfix", "target2")
	if err := fs.SetReference(conflictingRef); err != nil {
		t.Fatalf("Failed to set reference: %v", err)
	}
}

Additional Information

No response