rhg: upgrade `clap` dependency
This one is the worst one to upgrade since v2 -> v4 broke a ton of API,
which thankfully seems saner now.
Contrary to what was done in the `hg-core/src/examples/nodemap` rewrite,
we're not switching from the "builder" pattern to the "derive" pattern,
since that would imply a much larger diff. It can be done incrementally.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true