I think most users who change the set of immutable heads away from
`trunk() | tags()` are going to also want to change the default log
revset to include the newly mutable commit and to exclude the newly
immutable commits. So let's update the default log revset to use
`immutable_heads()` instead.
`test_templater` changed because we have overridden the set of
immutable commits there so `jj log` now includes the remote branch.