VCS: Do less when command is aborted internally

If the user cancels a command, output and error text might still be
interesting. When aborting a command internally (when Creator is closed,
or later when a command is re-executed for the same editor), we don't
want any of those, and they have a destructive potential.

Change-Id: I5e35fdf59c0fcdc0af45f13ac142da31edf18bd7
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>