extindex: preserve indexlevel=basic on incremental update

extindex: preserve indexlevel=basic on incremental update

-extindex needs to preserve indexlevel=basic when doing
incremental updates if the extindex was originally created with
indexlevel=basic.  Otherwise blindly upgrading somebody to
indexlevel=full would waste disk space and likely result in
inconsistent indexing on the Xapian side.

Fixes: bf2360b31 (extindex: support `-L basic' to avoid most Xapian space, 2025-08-13)