Faster dirty git status check (using git diff)

lib/git.zsh

@@ -13,7 +13,7 @@ parse_git_dirty() {
     if [[ $POST_1_7_2_GIT -gt 0 ]]; then
           SUBMODULE_SYNTAX="--ignore-submodules=dirty"
     fi
-    if [[ -n $(git status -s ${SUBMODULE_SYNTAX}  2> /dev/null) ]]; then
+    if [[ -n $(git diff --ignore-submodules HEAD 2> /dev/null) ]]; then
       echo "$ZSH_THEME_GIT_PROMPT_DIRTY"
     else
       echo "$ZSH_THEME_GIT_PROMPT_CLEAN"