Files that were new (staged additions) in the stash tree should
be staged when unstashing, even when not applying the index.

Ensure that when a file is added in the index and subsequently
modified in the working directory, the stashed working directory
tree contains the actual working directory contents.

We do not error on "merge conflicts"; on the contrary, merge conflicts
are a normal part of merging.  We only error on "checkout conflicts",
where a change exists in the index or the working directory that would
otherwise be overwritten by performing the checkout.

This *may* happen during merge (after the production of the new index
that we're going to checkout) but it could happen during any checkout.

The interesting one is the notification macro, which was returning
directly on a soft-abort instead of going through the cleanup.