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'

diff --cc contrib/completion/git-completion.bash
Simple merge

diff --cc t/t9902-completion.sh
Simple merge