heptapod-ci: use the new `--rust/--no-rust` flag to run the tests

run-tests: add option for running with and without Rust extensions This provide a simple and clear way to run the test with or without rust.

run-tests: rename "pure" variable to setup_opts There are more useful flag than just `--pure`. We clarify the variable name before adding more options.

setup-rust: add a --no-rust flag This new flag will make sure the rust extension will not be build. If neither `--rust` nor `--no-rust` is specified the `HGWITHRUSTEXT` is used.

cext: drop the WITH_RUST block These were used by the `direct-ffi` mode that just got dropped.

setup: use printf instead of print Yuya Nishihara pointed `printf` to me, so I will respect the cargo cult.

git: don't fail import when pygit2 is not install `test-duplicateoptions.py` was failing on py2 for be because I didn't have pygit2 installed. It failed because we depend on pygit2 at import time. This patch makes it so we successfully load the git extension even if pygit2 doesn't exist -- we just won't be able to use it in that case. Differential Revision: https://phab.mercurial-scm.org/D8268

hghave: add a check for pygit2 This will make it easy to use availability of pygit2 as in conditions such as `#if pygit2` and `(pygit2 !)`. It also makes it a little clearer why the test case skipped (when that happens). Differential Revision: https://phab.mercurial-scm.org/D8267