CRM-19198 Add form merge hook call to dedupe form

On batch merges the merge hook is called with type 'batch' and there is a chance to alter data. The form does not call this so the hook does not achieve the same results.

Calling it with 'batch' is a bit misleading so I am setting the context to 'form' but it works the same & it just means both cases need to be caught in the hook