Merge branch 'bc/completion-for-bash-3.0' into maint

Some people still use rather old versions of bash, which cannot grok
some constructs like 'printf -v varname' the prompt and completion
code started to use recently.

* bc/completion-for-bash-3.0:
  contrib/git-prompt.sh: handle missing 'printf -v' more gracefully
  t9902-completion.sh: old Bash still does not support array+=('') notation
  git-completion.bash: use correct Bash/Zsh array length syntax