Sync with upstream

Sync with upstream

* svnrdump@987530:
  svnrdump (load_cmd): r987513 doesn't do what it promises. Revert it.
  svnrdump (svnrdump__cmd_table): Remove one-letter subcommand aliases.
  svnrdump (load_cmd): Check that we can set revprops before driving the load editor.
  Tweak print statements in svnrdump.
  svnrdump.1: svnrdump does both dumping and loading now.
  dump_editor (open_file): Followup r986466 to fix a silly mistake.
  dump_editor (open_file): use svn_relpath_join.

* tests@987188:
  Make merge_tree_conflict.py 20 and 21 pass in single-db.
  Mark authz_tests.py 19 as Wimp for single-db.
  Add another helper method to the testing sandbox.