rhg: Fall back to Python for --version

Clap has some built-in support for printing something on --version,
but it looks different than what Pytho-based hg does.

Also, at the moment we’re not giving version numbers to rhg
separate from the Mercurial version.

Differential Revision: https://phab.mercurial-scm.org/D10144

rust/rhg/src/main.rs

@@ -29,6 +29,7 b' fn main_with_result('
 
     let app = App::new("rhg")
         .global_setting(AppSettings::AllowInvalidUtf8)
+        .global_setting(AppSettings::DisableVersion)
         .setting(AppSettings::SubcommandRequired)
         .setting(AppSettings::VersionlessSubcommands)
         .arg(