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/peepcode.zsh-theme

@@ -28,7 +28,7 @@ git_dirty() {
 }
 
 git_prompt() {
-  local cb=$(current_branch)
+  local cb=$(git_current_branch)
   if [ -n "$cb" ]; then
     local repo_path=$(git_repo_path)
     echo " %{$fg_bold[grey]%}$cb %{$fg[white]%}$(git_commit_id)%{$reset_color%}$(git_mode)$(git_dirty)"