Merge branch 'fc/completion-tests'

By Felipe Contreras (4) and others
* fc/completion-tests:
  completion: fix completion after 'git --option <TAB>'
  completion: avoid trailing space for --exec-path
  completion: add missing general options
  completion: simplify by using $prev
  completion: simplify __gitcomp_1
  tests: add tests for the __gitcomp() completion helper function
  tests: add initial bash completion tests