Merge branch 'sg/completion'

Merge branch 'sg/completion'

Clean-up and updates to command line completion (in contrib/).

* sg/completion: (22 commits)
  completion: restore removed line continuating backslash
  completion: cache the path to the repository
  completion: extract repository discovery from __gitdir()
  completion: don't guard git executions with __gitdir()
  completion: consolidate silencing errors from git commands
  completion: don't use __gitdir() for git commands
  completion: respect 'git -C <path>'
  rev-parse: add '--absolute-git-dir' option
  completion: fix completion after 'git -C <path>'
  completion: don't offer commands when 'git --opt' needs an argument
  completion: list short refs from a remote given as a URL
  completion: don't list 'HEAD' when trying refs completion outside of a repo
  completion: list refs from remote when remote's name matches a directory
  completion: respect 'git --git-dir=<path>' when listing remote refs
  completion: fix most spots not respecting 'git --git-dir=<path>'
  completion: ensure that the repository path given on the command line exists
  completion tests: add tests for the __git_refs() helper function
  completion tests: check __gitdir()'s output in the error cases
  completion tests: consolidate getting path of current working directory
  completion tests: make the $cur variable local to the test helper functions
  ...