New `--heptapod-hg-native` option
This option makes the functional tests create Mercurial native projects instead of the default type (currently `hg_git`). Hence it will be the main tool to validate HGitaly related developments. Later on, we might want to introduce dedicated CI jobs. Switching to parametrization can also be considered, but it wouldn't change so much in CI, because we now have tests sharding. The new flag is stored on the `Heptapod` object, for lack of better choice in the short term (arguably, that's not a property of the Heptapod server).