Merge branch 'fc/completion'

In addition to a user visible change to offer more options to cherry-pick,
generally cleans up and simplifies the code.

* fc/completion:
  completion: small optimization
  completion: inline __gitcomp_1 to its sole callsite
  completion: get rid of compgen
  completion: add __gitcomp_nl tests
  completion: add new __gitcompadd helper
  completion: get rid of empty COMPREPLY assignments
  completion: trivial test improvement
  completion: add more cherry-pick options