lib/git.zsh: Allow themes to opt-out of an expensive git status call

If neither DIRTY nor CLEAN is defined then this function is a noop,
so check up-front before making the call.

This speeds up prompts that empty out these values.

lib/git.zsh

@@ -8,6 +8,11 @@ function git_prompt_info() {
 
 # Checks if working tree is dirty
 parse_git_dirty() {
+  if [[ -z "$ZSH_THEME_GIT_PROMPT_DIRTY" &&
+        -z "$ZSH_THEME_GIT_PROMPT_CLEAN" ]]; then
+    return
+  fi
+
   local SUBMODULE_SYNTAX=''
   local GIT_STATUS=''
   local CLEAN_MESSAGE='nothing to commit (working directory clean)'