Merge branch 'ab/complete-show-all-commands'

The command line completion script (in contrib/) learned to
complete all Git subcommands, including the ones that are normally
hidden, when GIT_COMPLETION_SHOW_ALL_COMMANDS is used.

* ab/complete-show-all-commands:
  completion: add a GIT_COMPLETION_SHOW_ALL_COMMANDS
  completion tests: re-source git-completion.bash in a subshell

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

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