Move current_branch() from git plugin to core lib/git.zsh

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.

themes/sunrise.zsh-theme

@@ -1,6 +1,5 @@
 # Sunrise theme for oh-my-zsh
 # Intended to be used with Solarized: http://ethanschoonover.com/solarized
-# (Needs Git plugin for current_branch method)
 
 # Color shortcuts
 R=$fg_no_bold[red]