Branches: getting back to stable evolve for stable Mercurial

In particular, I got confirmation that the stable branch of evolve
is supposed to be compatible with the stable branch of Mercurial (and
not to be preparation for a potential evolve bugfix release).
Temporary incompatibilities of course always happen when a new release
candidate for Mercurial is pushed, bumping its stable branch: shortly
afterwards, the default branch of evolve is promoted to stable. Usually
we don't notice, but the incompatibility stayed a bit longer than usual
in the case of 5.7rc.
2 jobs for branch/default in 5 minutes and 26 seconds