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'
2c34e4e747