plugins: mercurial: Update the order prompt vars are displayed

If a user sets ZSH_THEME_HG_PROMPT_* as the following, he ends up with
%{$reset_color%} resetting the bold style of last character ")" because
_SUFFIX was expanded before calling hg_dirty():

    ZSH_THEME_HG_PROMPT_PREFIX="%{$fg_bold[blue]%}hg:(%{$fg[white]%}"
    ZSH_THEME_HG_PROMPT_SUFFIX="%{$reset_color%}"
    ZSH_THEME_HG_PROMPT_DIRTY="%{$fg[blue]%}) %{$fg[yellow]%}✗%{$reset_color%}"
    ZSH_THEME_HG_PROMPT_CLEAN="%{$fg[blue]%})"

With this patch, the variable ZSH_THEME_HG_PROMPT_SUFFIX is lastly
displayed, following the same order as in git_prompt_info(),
i.e. <prefix><dirty><suffix>.