fdo#75259: Let undo document share its string pool with the source doc.

We already do that for the pool helper.  It would only make sense to do
it for shared strings as well.

Change-Id: I813d262799af6f0c1d3fa12246ad973f852ac199
(cherry picked from commit e752620bf593af8839831f693ec21bb02a8b23d6)
Reviewed-on: https://gerrit.libreoffice.org/8210
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>