t9903: add tests for git-prompt pcmode

git-prompt.sh lacks tests for PROMPT_COMMAND mode.

Add tests for:
* pcmode prompt without colors
* pcmode prompt with colors for bash
* pcmode prompt with colors for zsh

Having these tests enables an upcoming refactor in
a safe way.
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.

With-suggestions-by: Øystein Walle <oystwa@gmail.com>
t9903: remove redundant tests

After refactoring __git_ps1_colorize_gitstring, codepaths for bash and zsh
became mostly common and tests for bash and zsh became redundant.

Remove tests for zsh. Keep one minimal test that stress the difference
in codepaths for bash and zsh.

Suggested-by: SZEDER Gábor <szeder@ira.uka.de>
git-prompt.sh: do not print duplicate clean color code

Do not print a duplicate clean color code when there
is no other indicators other than the current branch
in colored prompt.

Acked-by: SZEDER Gábor <szeder@ira.uka.de>
git-prompt.sh: add missing information in comments

Mention that the command below is needed for prompt
in ZSH with PS1:
  setopt PROMPT_SUBST

Make it clear that colored prompt is only available
in PROMPT_COMMAND mode.