lib/git.zsh: Added git_commits_behind function (#4450)

* Added git_commits_behind function
* Added 'command' to git_commits_behind function
* git_commits_behind code review changes

lib/git.zsh

@@ -83,6 +83,13 @@ function git_commits_ahead() {
   fi
 }
 
+# Gets the number of commits behind remote
+function git_commits_behind() {
+  if $(command git rev-parse --git-dir > /dev/null 2>&1); then
+    echo $(git rev-list --count HEAD..@{upstream})
+  fi
+}
+
 # Outputs if current branch is ahead of remote
 function git_prompt_ahead() {
   if [[ -n "$(command git rev-list origin/$(git_current_branch)..HEAD 2> /dev/null)" ]]; then