Tweak checking branches in Project#open_branches

This changes 4 things:

1. Project#protected_branches_names has been renamed to
   Project#protected_branch_names.

2. Project#open_branches uses a Set for the branch names as checking
   values in a Set is faster than checking values in a (large) Array.

3. Some redundant code in Project#open_branches has been removed.

4. Project#protected_branch_names now uses #pluck instead of #map,
   removing the need for loading entire DB records into memory.