Membership sharing doesn't work after merge (regression)

This is a regression of CRM-12695.

When membership is shared through relationships, the sharing does not get updated for new relationships after a merge.

Steps to recreate:

Create an organisation: 'Membersonly Organisation'
Create another organisation: 'Main organisation'
Create another organisation: 'Duplicate organisation'

Create contacts:
'Employee one'
'Employee two'
'Employee three'
'Employee four'

Create a new membership type:
Name: TEST Membership cascades
Org: Membersonly organisation
Ensure that 'relationship type' is set to 'Employer of' (so the membership should cascade to employees)

Add membership to main org:
Add a 'TEST membership cascades' to 'Main organisation'

Add employment relationships:
Employee one > Main organisation
Employee two > Main organisation
Employee three > Duplicate organisation
Employee four > Duplicate organisation

Merge the 'Duplicate organisation' into the 'Main organisation' (move related relationship)

Expected result:
'Main organisation' has all four employees, and all employees have the 'TEST membership cascades' on their records

Actual result:
'Main organisation' has all four employees, but only 'Employee one' and 'Employee four' have the membership? It has been lost from 'Employee two' and was not added to 'Employee three'.

Solution:

Fix and implement tests to prevent future regression. Rather than a webtest, lets have a unit test for this as part of the merging code - we have unit tests on that now for other parts of functionality.