xmerge: use String.format

http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/String.html#format(java.lang.String, java.lang.Object...)

Change-Id: I98b7fca9dee870c9a1d0f9842cbb009a13537967
Reviewed-on: https://gerrit.libreoffice.org/11995
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>