Merge branch 'rh/complete-symbolic-ref'

Command line completion script (in contrib/) learned to complete "git symbolic-ref" a bit better (you need to enable plumbing commands to be completed with GIT_COMPLETION_SHOW_ALL_COMMANDS). * rh/complete-symbolic-ref: completion: add docs on how to add subcommand completions completion: improve docs for using __git_complete completion: add 'symbolic-ref'