rhg: Use clap’s support for global CLI arguments By default, clap only accepts app-level arguments (as opposed to sub-command level) to be specified before a sub-command: `rhg -R ./foo log`. Specifying them after would be rejected: `rhg log -R ./foo`. Previously we worked around that by registering global arguments both at the app level and on each sub-command, but that required looking for their value in two places. It turns out that Clap has built-in support for what we want to do, so let’s use it. Also, Clap "settings" turn out to be either global or not too. Let’s make `AllowInvalidUtf8` apply to sub-commands too. Differential Revision: https://phab.mercurial-scm.org/D10080