Mercurial cloning: no share-safe

Because the default Mercurial version provided by the Runner itself (usually as part of the Runner Helper image) is now 6.7, it creates repositories that require share-safe by default. This breaks job scripts that are using Mercurial<5.7 (this happened to the unit-golang job of this very project).

On the other hand, shares are seldom used with the kind of CI provided by Heptapod Runner. Still, users that need them can be advised to include format.use-share-safe.automatic-upgrade-of-mismatching-repositories=yes in their scripts. Subsequent runs will have no problem to pull/update a repo that has switched to share-safe.