Make merge_tree_conflict.py 20 and 21 pass in single-db. A missing
directory will have full metadata in single-db, so children also show as
missing.
* subversion/tests/cmdline/merge_tree_conflict_tests.py
(tree_conflicts_merge_edit_onto_missing,
(tree_conflicts_merge_del_onto_missing): Build sandbox, tweak expectations
for single-db.
* subversion/tests/cmdline/svntest/actions.py
(deep_trees_run_tests_scheme_for_merge): Only build sandbox if not built.
git-svn-id: http://svn.apache.org/repos/asf/subversion/trunk/subversion/tests/cmdline@987188 13f79535-47bb-0310-9956-ffa450edef68