feat: plain color branch name when hide-dirty set

If hide-dirty is set, parse_git_dirty will not check the directory's
status. Rather than coloring the git-status portion of the prompt green
(which typically indicates a clean working directory) we color this
portion of the prompt white to reflect the lack of information.

themes/agnoster.zsh-theme

@@ -112,6 +112,8 @@ prompt_git() {
     ref=$(git symbolic-ref HEAD 2> /dev/null) || ref="➦ $(git rev-parse --short HEAD 2> /dev/null)"
     if [[ -n $dirty ]]; then
       prompt_segment yellow black
+    elif [[ "$(git config --get oh-my-zsh.hide-dirty 2>/dev/null)" = 1 ]]; then
+      prompt_segment white black
     else
       prompt_segment green $CURRENT_FG
     fi