Merge References for Statements in wbmergeitems

If statements have the same main snak and qualifier
hashs when merging then rather than create a new
statement we should just merge the references accross!

Bug: 58850
Change-Id: I256c3e757f7bed8f2e3b731e9ad5f26d507690d3
Merge "Merge References for Statements in wbmergeitems"