Provide a test for heptapod#72

The simplest way to do it could be to introduce an extension in hg-git/heptapod_hooks that would dump HEPTAPOD prefixed entries from ui.environ, because then, we have everything needed to activate it from the functional tests Project.extend_hgrc.

Probably won't have time to do it before Heptapod 0.6.0, hence the issue.

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