xmerge: Convert a primitive type into a string

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

Change-Id: If361383be52d8258be7c8c9bddf3ba05d5ca92c1
Reviewed-on: https://gerrit.libreoffice.org/11026
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>