git-prompt.sh: refactor colored prompt code

git-prompt.sh: refactor colored prompt code

__git_ps1_colorize_gitstring() sets color codes and
builds the prompt gitstring. It has duplicated code
to handle color codes for bash and zsh shells.
__git_ps1() also has duplicated logic to build the
prompt gitstring.

Remove duplication of logic to build gitstring in
__git_ps1_colorize_gitstring() and __git_ps1().

Leave in __git_ps1_colorize_gitstring() only logic
to set color codes.

Signed-off-by: Eduardo R. D'Avila <erdavila@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>