Loading session and not merging also deletes built-in variables

[Felix Krull]

When loading a session and not merging it with the current session, the built-in variables (so pi, e, all the units) are missing. Observe:

1. Save a session.
2. Load the session.
3. When asked whether you want to merge the session with the current one, select "No."
4. The built-in variables are now gone.

[Pol Welter]

As of commit fd349de.

[Helder Correia]

@fk Please confirm whether it works for you now.

[Felix Krull]

Yup, that fixed it.