Topic/heptapod/fix tests hg4.9

With this branch, all the tests pass for me with Mercurial 4.9

This MR mostly for information, I intend to publish in a few days at most. Noticed while working on this that we had a similar topic by @madprog for 4.6 that never got published.

The topic starts by a rebase on the current upstream default branch.

Note in particular that I did restrict the supported Mercurial version drastically in the currently latest commit. I find it safer that way for the Heptapod context. People may disagree.