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

localrepo: add git extension to autoextensions list Differential Revision: https://phab.mercurial-scm.org/D8266

gitlog: implement __iter__ on changelog This allows `hg phase --force --draft -r .` to work, for example. Differential Revision: https://phab.mercurial-scm.org/D8264

gitlog: fix embarassing bug that prevented log from showing correct parents We had the correct parents in the database, but the changelog was always returning p1 for all parents. Oops. Differential Revision: https://phab.mercurial-scm.org/D8263