hg: don't reuse repo instance after unshare()
Unsharing a repository is a pretty invasive procedure and fundamentally
changes the behavior of the repository.
Currently, hg.unshare() calls into localrepository.__init__ to
re-initialize the repository instance. This is a bit hacky. And
future commits that refactor how localrepository instances are
constructed will make this difficult to support.
This commit changes unshare() so it constructs a new repo instance
once the unshare I/O has completed. It then poisons the old repo
instance so any further use will result in error.
Surprisingly, nothing in core appears to access a repo instance
after it has been unshared!
.. api::
``hg.unshare()`` now poisons the repo instance so it can't be used.
It also returns a new repo instance suitable for interacting with
the unshared repository.
Differential Revision: https://phab.mercurial-scm.org/D4557