Return early if folder isn't a git repo

Minor performance improvement

prompt.zsh

@@ -34,6 +34,7 @@ zstyle ':vcs_info:git*' actionformats ' %b|%a'
 
 # Fastest possible way to check if repo is dirty
 git_dirty() {
+	[ -d .git ] || return
 	git diff --quiet --ignore-submodules HEAD 2>/dev/null; [ $? -eq 1 ] && echo '*'
 }