add function to determine current upstream

for git_prompt_ahead() (in lib/git.zsh) to work with remotes different
than 'origin', we need a way to determine the origin of the currently
active branch.

This uses git-config to find the current branches remote (if any)

plugins/git/git.plugin.zsh

@@ -52,6 +52,11 @@ function current_branch() {
   echo ${ref#refs/heads/}
 }
 
+function git_current_upstream(){
+  local upstream=$(git config --get branch."$(current_branch)".remote) || return
+  echo $upstream
+}
+
 # these aliases take advantage of the previous function
 alias ggpull='git pull origin $(current_branch)'
 compdef ggpull=git