Merged documents when saved as individual documents, the filenames
start from 0. This patch provides a fix for this as it introduces
natural numbering of documents (starting from 1) rather than the
previous 0-based numbering when saving individual merged documents.
e.g. mergedfile_1, mergedfile_2 and so on.

Change-Id: I67275656455788bb5ae935d7d42a349aa0510b36
Reviewed-on: https://gerrit.libreoffice.org/66658Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>