xmerge: Convert a primitive type into a string

Avoid unnecessary temporaries when converting primitive data types into a String.

Change-Id: Ied91859e756ff392b1a0ea8d6d889afae11b2f8d
Reviewed-on: https://gerrit.libreoffice.org/11366
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>