xmerge: remove String constructor invocation

Change-Id: Ibd7a94c0f53be3b3a3e9ee764dbdeb6199857441
Reviewed-on: https://gerrit.libreoffice.org/10576
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>