rhg: add a limited `rhg root` subcommand

Authored by acezar.


rhg: add a limited rhg root subcommand

Clap has been choosen for argument parsing for the following reasons:

  • it's a wildly used and maintained crate
  • it can deal with OS encoding making it suitable for any encoding
  • it supports nonambiguous prefix matching as already available in hg
  • it will soon allow for structopts-style declarative pattern instead of the currently used builder pattern

Differential Revision: https://phab.mercurial-scm.org/D8613