Makes it clear this is only used in dropdowns, instead of cluttering up Project class. Since we only care about branch names, it is also possible to refactor out a lot of the set/reject logic.

A benchmark on Array/Set subtraction favoured using Arrays. This was with 5000 ‘branches’ and 2000 ‘protections’ to ensure a similar comparison to the commit which introduced using Set for intersection.

Comparison:
   array subtraction:      485.8 i/s
     set subtraction:      128.7 i/s - 3.78x slower