cli: expose --version on the root command

The flag was disabled in the initial commit, leaving the binary with
no way to report its number — wrong for a release. clap scopes the
automatic version flag to the root command (-V/--version), so the
per-subcommand -v target-version options of pull and chlog are
unaffected.

src/main.rs

@@ -31,7 +31,6 @@ fn main() {
     LogWrapper::new(multi.clone(), logger).try_init().unwrap();
     let matches = command!()
         .subcommand_required(true)
-        .disable_version_flag(true)
         .subcommand(
             Command::new("new")
                 .about("Scaffold a new Debian source package (buildable right away)")