WIP Fully native Mercurial without Git: diversions

These shortcuts are enough to make the functional tests pass, but
some of them are obviously unacceptable, while some other ones
can be kept until the corresponding feature is ready for implementation.

This changeset is supposed to shrink until there are only acceptable
diversions, at which point we can consider publishing it and go
to the next step: more testing of the `hgitaly2` branch and final merge
into the main `heptapod` development branch.

branch : hgitaly2
4 jobs for !290 with topic/heptapod/no-git-conversion in 10 minutes and 18 seconds (queued for 3 seconds)
merge request