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The ruby prompt info was not interpolating properly. Switching to use
 ruby_prompt_info helper. This addresses the issue.

Fixes #4085: core -> plugin dependency issue.
Rename it to git_current_branch for clarity that it's git-specific.
Update all plugins that were calling it to use new name.
Fix variable leaks by making more variables in lib/git.zsh local.
Have lib/git.zsh use [[ ]] instead of [ ] everywhere.