rhg: upgrade dependencies This upgrades all dependencies to their latest version, except `clap`, which is upgraded to the last 2.x series version, since 3.x does not support our minimum supported Rust version of 1.48.0. This contains a security fix for `regex` which does not affect us too much, but doesn't hurt, and the rest of the upgrades are there simply to keep up. Differential Revision: https://phab.mercurial-scm.org/D12357