prompt: factor out gitstring coloring logic

So that we can extend it with ZSH-colors in a later patch.

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

Add colors suitable for use in the ZSH prompt.  Having learnt that the
ZSH equivalent of PROMPT_COMMAND is precmd (), you can now use
GIT_PS1_SHOWCOLORHINTS with ZSH.

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