prompt: introduce GIT_PS1_STATESEPARATOR

A typical prompt looks like:

    artagnon|master *=:~/src/git$
                   ^
                   why do we have this space?

Nobody has branch names that end with +, *, =, < or > anyway, so it
doesn't serve the purpose of disambiguation.

Make this separator configurable via GIT_PS1_STATESEPARATOR.  This means
that you can set it to "" and get this prompt:

    artagnon|master*=:~/src/git$

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