prune: spell --successor flag without any unnecessary shortcuts If a user wants to spell out -s, it makes sense to allow that. Before this patch, prune would complain that --successor is not a recognized option. Obviously people don't usually need to spell --successors by hand thanks to shell completion (at least for Bash) using debugcomplete to see all available flags, so this patch doesn't bring any need for more typing. And thanks to Mercurial understanding shortened forms of command-line flags as long as they are unambiguous, the old-style `--succ` flags still work normally, and there are tests that use them. But two tests now use the full form to demonstrate that both ways work.