Add git_prompt_behind and git_prompt_exists

This adds two new theme functions for git:
* `git_prompt_behind` works in a identical fashion to `git_prompt_ahead`
  and will output a format variable (`ZSH_THEME_GIT_PROMPT_BEHIND`) if
  the branch is behind.
* `git_prompt_remote` will output one format variable if the branch
  exists on remote (`ZSH_THEME_GIT_PROMPT_REMOTE_EXISTS`), and another
  if the branch is unpublished (`ZSH_THEME_GIT_PROMPT_REMOTE_MISSING`).

The old `git_prompt_ahead` has been changed. Using git log is subject
to formatting in .gitconfig, which can be overridden and will break
this function. Relying on rev-list is much more stable.

lib/git.zsh

@@ -59,13 +59,6 @@ git_remote_status() {
     fi
 }
 
-# Checks if there are commits ahead from remote
-function git_prompt_ahead() {
-  if $(echo "$(command git log @{upstream}..HEAD 2> /dev/null)" | grep '^commit' &> /dev/null); then
-    echo "$ZSH_THEME_GIT_PROMPT_AHEAD"
-  fi
-}
-
 # Gets the number of commits ahead from remote
 function git_commits_ahead() {
   if $(echo "$(command git log @{upstream}..HEAD 2> /dev/null)" | grep '^commit' &> /dev/null); then
@@ -74,6 +67,29 @@ function git_commits_ahead() {
   fi
 }
 
+# Outputs if current branch is ahead of remote
+function git_prompt_ahead() {
+  if [[ -n "$(command git rev-list origin/$(current_branch)..HEAD 2> /dev/null)" ]]; then
+    echo "$ZSH_THEME_GIT_PROMPT_AHEAD"
+  fi
+}
+
+# Outputs if current branch is behind remote
+function git_prompt_behind() {
+  if [[ -n "$(command git rev-list HEAD..origin/$(current_branch) 2> /dev/null)" ]]; then
+    echo "$ZSH_THEME_GIT_PROMPT_BEHIND"
+  fi
+}
+
+# Outputs if current branch exists on remote or not
+function git_prompt_remote() {
+  if [[ -n "$(command git show-ref origin/$(current_branch) 2> /dev/null)" ]]; then
+    echo "$ZSH_THEME_GIT_PROMPT_REMOTE_EXISTS"
+  else
+    echo "$ZSH_THEME_GIT_PROMPT_REMOTE_MISSING"
+  fi
+}
+
 # Formats prompt string for current git commit short SHA
 function git_prompt_short_sha() {
   SHA=$(command git rev-parse --short HEAD 2> /dev/null) && echo "$ZSH_THEME_GIT_PROMPT_SHA_BEFORE$SHA$ZSH_THEME_GIT_PROMPT_SHA_AFTER"