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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.

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.