Fixed diff generator when several identical tables or columns were renamed

Fixed diff generator when databases have several tables with identical
DDL and several of them were renamed.
Fixed diff generator when two tables have several columns with
identical DDL and several of them were renamed.
Port PR https://github.com/propelorm/Propel/pull/839 from Propel1.
Merge pull request #566 from javer/patch-diff-samecolumns

Fixed diff generator when several identical tables or columns were renamed