Add debug documents to SwDBManager::MergeMailFiles

Just like the debug documents in SwDBManager::MergeDocuments.

Change-Id: I275575853c80d4e19d6df5ba9d8dc689353d7c85
Reviewed-on: https://gerrit.libreoffice.org/10970
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>