README: document requirement for builtin zstd Differential Revision: https://phab.mercurial-scm.org/D10226

ci: Add a job testing with rhg installed as `hg` This significantly increases test coverage of rhg, without duplicating many tests that already exist. The `cargo build` command being remove only compiled a `rhg` executable (as shown by the preceding `cd` command) but since a previous patch `run-tests.py --rhg` now takes care of that. Differential Revision: https://phab.mercurial-scm.org/D10196

run-test: install rhg if --rhg is passed Before this, --rhg was only working with --local. Differential Revision: https://phab.mercurial-scm.org/D10195

makefile: add a install option This gives and easy way to install rhg that we can use in `run-test.py` in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D10194

makefile: add a build-rhg option This gives an easy action to build the rhg-binary. This will be useful for the `install-rhg` action in the next changeset. Differential Revision: https://phab.mercurial-scm.org/D10193

makefile: add a install-chg option This is done as a gratuitous improvement on the way to add makefile entry to build and install rhg. It seems saner to have equivalent entry for chg too. Differential Revision: https://phab.mercurial-scm.org/D10192

makefile: add a build-chg option This is done as a gratuitous improvement on the way to add makefile entry to build and install rhg. It seems saner to have equivalent entry for chg too. Differential Revision: https://phab.mercurial-scm.org/D10191