Speed up rehash process when there are many Ruby versions
On my system that has 25 versions under rbenv, this speeds up rehash
almost 3-fold:
- before: 391 ms
- after: 134 ms
This is achieved by removing duplicate names of executables before
registering them as shims. Since most Rubies will share a lot of the
same executable names ("ruby", "rake", "bundle", ...), this is a
considerable reduction in number of shims registered.