Forbid obsolete references (tags and bookmarks)
Nothing prevents in Heptapod tags and bookmarks to point to obsolete changesets.
It is becoming an important assumption, especially in HGitaly that no GitLab "revision" except explicit hashes can point to an obsolete changeset.
Besides it's rather easy to shoot oneself in the foot with tags and obsolescence:
echo fo > foo
hg commit -Am "foo"
hg tag "1.0"
# oops there was something wrong
echo foo > foo
hg absorb # ka-boom! the 1.0 tag now points to an obsolete changeset
So it would be nice of Heptapod to protect users from that. The only problem is that users may already have that kind of situation (perhaps with tags years old) and we shouldn't block them for that.
Heptapod 0.18.0 is a good opportunity to add this kind of protection.