t7003: ensure --prune-empty removes entire branch when applicable

Sanity check before changing the logic in git_commit_non_empty_tree.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7003-filter-branch.sh

@@ -371,6 +371,13 @@ test_expect_failure '--prune-empty is able to prune root commit' '
 	test_cmp expect actual
 '
 
+test_expect_failure '--prune-empty is able to prune entire branch' '
+	git branch prune-entire B &&
+	git filter-branch -f --prune-empty --index-filter "git update-index --remove A.t B.t" prune-entire &&
+	test_path_is_missing .git/refs/heads/prune-entire &&
+	test_must_fail git reflog exists refs/heads/prune-entire
+'
+
 test_expect_success '--remap-to-ancestor with filename filters' '
 	git checkout master &&
 	git reset --hard A &&