upgrade: more standard creation of the temporary repository By using the standard path to create a repository we fill some hole in the current initialization process. The one who triggered this changeset was the lack of extensions initialization.

upgrade: use the repository 'ui' as the base for the new repository The `repo.baseui` contains all the configuration but the one specific to the repository (so it can be used when dealing with local peer and sub- repository). However, we need the repository config to be taken into account when doing the upgrade. Otherwise, the upgrade related config that exists in the repository config won't be taken into account when performing the update. A buggy and surprising behavior. We had to work around protection set around `repo.ui.copy` since we are an uncommon case.

upgrade: add a test to show the repository config being ignored The upgrade process ignores the config within the repository. The next changeset fixes it, but we introduce this test before to show it actually tests our target.