Merge branch 'pb/complete-config'

The command line completion script (in contrib/) learned to
complete configuration variable names better.

* pb/complete-config:
  completion: add and use __git_compute_second_level_config_vars_for_section
  completion: add and use __git_compute_first_level_config_vars_for_section
  completion: complete 'submodule.*' config variables
  completion: add space after config variable names also in Bash 3

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

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