agnoster: speedup for repos with enabled hide-dirty

For huge repos, status checking can take many seconds. It's pretty annoying to
have such delay on each prompt message inside such repo. agnoster uses
parse_git_drity which respect the oh-my-zsh.hide-dirty flag. Thus, we don't
perform an expensive check in this case. However, it doesn't solve the original
problem because we still have a delay because of check-for-changes. The line
should be executed only for dirty repos.

themes/agnoster.zsh-theme

@@ -117,7 +117,9 @@ prompt_git() {
 
     zstyle ':vcs_info:*' enable git
     zstyle ':vcs_info:*' get-revision true
-    zstyle ':vcs_info:*' check-for-changes true
+    if [[ -n $dirty ]]; then
+      zstyle ':vcs_info:*' check-for-changes true
+    fi
     zstyle ':vcs_info:*' stagedstr '✚'
     zstyle ':vcs_info:*' unstagedstr '●'
     zstyle ':vcs_info:*' formats ' %u%c'