xmerge: String comparison

Change-Id: Icda2e693c1e0177708c83af8048bc4e56c0db4ce
Reviewed-on: https://gerrit.libreoffice.org/10665
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>