Check for "new" email addresses in pre-push checks

[cburkert]

Would be nice if git-privacy could warn if someone accidentally committed with a different email address than usual.
This happens for instance if one contributes from a different setup than usual with a different global Git configuration.
A detection would prevent unwanted usage of, e.g., private email addresses.
Maybe other hooks than pre-push are better suited.

The detection could alert if

• the address is not part of the history already AND
• it is read from a non-local user.email config.