rhg: consistently use the command name given in clap::command!(<...>) macro

Before this patch there are 2 things the user controls:

1. the module/command name, specified in subcommand! macro
2. the command name, specified in clap::command! macro

If these are out of sync, we get no compile error or a clear runtime
error, but instead a confusing behavior where command line parser
parses one thing, but running it doesn't work.

This commit makes the clap::command! macro the sole authority
determining the command name, so we don't have to worry about
this weird behavior any more.

It also makes it easy to validate agreement between (1) and (2)
if we want it, but I didn't add the check because I'm not sure
people necessarily want it.

rust/rhg/src/main.rs

@@ -545,13 +545,15 struct SubCommand {
 }
 
 macro_rules! subcommand {
-    ($command: ident) => {
+    ($command: ident) => {{
+        let args = commands::$command::args();
+        let name = args.get_name().to_string();
         SubCommand {
-            args: commands::$command::args(),
+            args,
             run: commands::$command::run,
-            name: stringify!($command).to_string(),
+            name,
         }
-    };
+    }};
 }
 fn subcommands() -> Vec<SubCommand> {
     vec![