Changes themes displaying RVM or other Ruby version info to use the central
ruby_prompt_info function. This supports more Ruby versioning mechanisms,
reduces copy-and-paste code, and avoids "zsh: no such file or directory: rvm-prompt"
when run on machines that do not have RVM installed.
Changes the prefix/suffix variable names to ZSH_THEME_RUBY_PROMPT_PREFIX and
ZSH_THEME_RUBY_PROMPT_SUFFIX, since they apply to all Ruby versioning mechanisms,
not just RVM.
Allows empty ZSH_THEME_RUBY_PROMPT_PREFIX and ZSH_THEME_RUBY_PROMPT_SUFFIX.
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.
