The script can be re-run (in some cases at least!) to fix up incomplete migrations...

The script can be re-run (in some cases at least!) to fix up incomplete migrations (see bug 34981 for enwiki)

There's little point reading a row from the db if rev_parent_id != null, as the parent id has already been populated.

Added this as an extra condition, filtering on this after limiting to a relatively small batch should be ok (rev_parent_id isn't indexed)

Change-Id: I85b5813ab214869350c95f3bee58daca3e92eba0