git-prompt.zsh: introduce thin ZSH wrapper

To facilitate a colored prompt in ZSH, write a thin wrapper around
git-prompt.sh, factoring out and overriding the coloring logic.  Since
ZSH lacks a PROMPT_COMMAND, instruct the user to execute __git_ps1
inside precmd().

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>