There was a weird asymmetry, where the undo-commands would
register the fulltext index of the dive, but the core would
unregister the fulltext index in the "unregister_dive()"
function.
To make this more logical, create a "register_dive()" function
in core that does registers the fulltext index.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>