Add tests for git-prompt PROMPT_COMMAND mode

Adding tests for:
- non-color PROMPT_COMMAND mode
- bash color PROMPT_COMMAND mode
- zsh color PROMPT_COMMAND mode
Refactor git-prompt

Remove logic to build gitstring out of __git_ps1_colorize_gitstring and
leave only color coding logic.
The logic to build gitstring in __git_ps1 for non-colored prompt is
now also used for colored prompt.
Allow usage of colored prompt in PS1 mode